If I want to have a really tender beef roast what temperature should I cook at and how long?

For slicing?, deli style or more like pot roast?
Usually you cook a roast beef at about 350 till the internal temp is 130-140, that should get you med rare to medium slicing beef.
Braising at low temp in the oven will get you tender beef providing you use the right cut, like a chuck roast. That's all I got, lol

I agree about braising. Like three hours in a Dutch oven at around 300. I usually sear mine first with a little butter and oil. Take the roast out, sear my carrots and onion (thick slices), deglaze the pan with a dry red wine. Put the roast back in with some of the onions . Add some beef broth and a half packet of au jus mix. Braise for an hour and a half. Then add the rest of the onions, carrots, peeled turnip chunks/potatoes. I also like to use a bundle of thyme when I first start braising the roast. Perfect every time

I plan on slicing and making hot beef sandwiches.

If the roast is tougher than you like, slice it thin. All the delis here roast tough top rounds for their deli beef, cooked rarish, and cut thin it seems tender.

I am not much of a cook, but would marinating/brining help tenderize a less than ideal cut?

I know it helps keep turkey moist. And soaking in milk makes squirrel less tough.

it depends on the cut of meat if it will slice much for you , mine mostly falls apart

but it is good
salt and pepper the meat.
2-3 onions halved and sliced.
little lard in a cast iron dutch oven , get it to smoke .
with a tongs drop the meat in and get a sear on all sides.
pull the meat out a little more lard , then drop the onions in.
as soon as the edges start to brown up add water start by adding enough to get a good sizzle and deglaze that goodness off the bottom of the pan , and work it around deglazing , when you have it deglzed and a nice broth going add your meat first and any veg you want to cook if you have any.
water about half way up the meat.
Lid on , 350 for 40 minutes drop to 275-300 and go about an hour a pound , check it after each hour , if you want it to slice probably stop when you have an internal temp of 145-150

I haven't tried that I am usually using a lesser cut of meat a peice that would be tough.

I keep going and it makes for more of a pulled beef stop when the fat is all rendered out.
you won't have water left in the end you can often smell it right about the time you run out of water

water can't get hotter than 212 by much unless it is under pressure , even a dutch oven lid isn't enough weight to get much pressure so it slows the cooking till near the end letting the heat get to the middle before overcooking the outside.

a good fatty peice of meat like brisket or shoulder it is very nice.

you can pull it all apart and let it soak a bit in the rendered fat in the and it takes back on the juices.

the added time breaks down the bonds in tough meat , the cheap meat.

you can spice it up how you like , some add peprochini to make it more of an Italian beef.

Depends on what you are trying to do...
So like a ribeye roast at Christmas that you want medium rare is all temperature with a meat thermometer in the thickest spot.
Doing like a pork butt for pulled pork you don't cook to that internal temp you cook low and slow to surpass that point to the temp that it starts to break down.

If you are just trying to do a simple roast this is what you want to do...
thaw roast all day in fridge
let set out over night to warm
in morning put it in the crock pot on low. (sear the edges in a screaming hot pan of avocado oil if you want (avocado oil can be really hot))
cover in broth, add 3 tbl garlic, add onion soup mix powder, and a brown gravy packet, fine dice carrots and celery including leaves, and onions.... just flavoring
3 or 4 whole dry bay leaves. Don't skip this.
let that cook all day and about two hours before supper.
cut and add chunky carrots, chunky potatoes, chunky onions... the trick is to get them all the same size chunks so they cook the same amount.
add a palm full of salt and about a half a palm full of pepper.
let that go about an hour and try a chuck of veggies... when they are the softness you like its time to eat.

I would put 325 for hours

Crockpot on high
Fall apart 10 hours
Slice. 7 hours

Unless you’re bored and want to watch marinade and all that

Coat with lawrys seasoning salt. Mix up pack of McCormick mushroom gravy and pour over it. Cook at above times. I cook another pack of gravy to put on it while eating mmmm

Ohio boy , I forgot to say Bay leaf , but yes I use several the jar of them is over the stove 3,4,5,6,7 sort of a measure by feeling.

bay leaf is some sort of cooking flavor magic , looks like nothing but adds good flavor

The wife has a bay tree in the house, and jars full of dried leaves for cooking. Some of the leaves are pushing 6 inches long.

Season with salt/pepper, sear that bad boy in a hot pan with a couple tablespoons of olive oil, transfer to a roasting pan, dump a packet of dry onion soup mix over it, then pour a can of coke over it, cover and bake at 325 for about 30 minutes per pound of meat. Usually, 2-1/2 to 3 hours. The coke will tenderize shoe leather, I do believe. You will be shocked it will not taste like you cooked it in coke at all.

Cow, Pig or wild game: Cook moist, low and slow.

14 # Beef roast open pit 6 hrs. over lump charcoal and mesquite, rub with S&P for hot beef samis or otherwise. That way you can add seasonings as you want later when you eat it or make other dishes. Main things are to start it at room temp, DO NOT POKE IT WITH UTENSILS!, and when done let IT REST COVERED with foil for 10 mins per pound then slice it AGAINST THE GRAIN. Enjoy :-)

Get a 3-5 lb Sirloin Tip Roast.

Cover with kosher salt and Fresh cracked pepper and let it set ..uncovered in the fridge for 24-48 hours.

Set oven on 250 and cook until internal temp is 130

Let rest for 15 minutes..on the counter..on a plate

Heat oven to 475... Throw roast back in for 10 minutes or until it has a nice browned finish.

Remove and slice

No one has asked what kind of roast, so every answer here could be correct, or completely wrong. A chuck roast and a bottom round roast need to be cooked very differently.

I like cooking roast this way too. At the half way point, I like to flip the roast over is the gravy/broth is only half way up the roast.