Beef topside roast, can I cook it in.

an ovenbag in the crockpot? Never done one before and as oven will be in use for other things was wondering if I could do it in the crockpot. Also do I need to add anything else to it? Thank you in advance to those who post helpful and useful comments.

Not much help with the oven bag and crock pot here as i've never cooked it that way. If you're not using the top of the oven though. i've always cooked topside by browning all sides at high heat in a tiny bit of dripping in a heavy thick based pot. When browned I add about a 1/4 cup of water (carefully as it will steam) and plonk a tight fitting lid on and turn down to a low simmer.
Check every so often so it doesn't burn out and top up with a little more water ( you get lovely brown gooey scrapings for making gravy) so keep water to a minimum as you're not boiling it. can't tell you exact time, but I think I do about 1 1/2 kg for about two hours.
It should be falling apart when you take it out to rest. i do like my meat well done so you may prefer it rarer. Let it rest in tinfoil and make lovely rich gravy from the bottom of the pot.
I'm drooling thinking about it lol.
Please bear in mind, I'm not as expert as some in here so others may have better methods.

Yes roast beef in an oven bag in the corck pot is beautiful and tender. long and slow is the secret

This is an old recipe given by listeners of Radio in NZ and put together by a young Alison Holst.

It is very nice and a long keeper.

Topside Supreme

3 large onions sliced
3 tablespoons fat
1 to 1 ½ kg topside of beef
2 tablespoons flour
1 tsp salt
¼ tsp pepper
¼ tsp thyme
2 cups water
1 tablespoon beef stock powder
2 tablespoons vinegar
1 tablespoon Worcestershire sauce

Brown onions slowly in the fat in a large heavy bottomed saucepan.
Add meat and turn to seal in all juices.
Mix flour and dry ingredients, sprinkle over meat then add water and stock, vinegar, and Worcestershire sauce.
Cover lightly.
Simmer over very low heat until meat is tender, 2 – 3 hours depending on size of meat.

Can be done in a slow cooker after the browning before adding dry ingredients and the liquids.

You shouldn't need an oven bag in a crockpot. just put in plenty of what you like brown the meat and turn on low all day. I add finely chopped onion and garlic as it makes a lovely sweet gravy I do this sometimes for a lunch and serve hot roast beef sandwiches with horse Radish sauce and crisp green salad always a hit :)

Also one of those meats on special occasionally to very cheap and it freezes well so stock up, same for chuck roasts.

Valentinos recipe looks great as above I don't add any liquids as there are enough juices anyway but that's the beauty of a crockpot or slow cooker almost anything works very well only thing you can get wrong is the seasoning.

Thank you so much for all your replies, they have been a great help. I must admit to being a bit of a learner with cooking different meat. I'm a baker rather than a cook. Can do mince, chicken, roast lamb, sausages, casseroles and that's about it. Hopeless with steak and chops, but then my son doesn't like them, would prefer mince or roast, most likely because I can not cook steak well at all. I also have to be careful as to what I add in the way of seasonings etc as he reacts to so many things.

Beaker raised and reminded me of liquids when cooking in a slow cooker, true the slow cooker keeps it liquids hence a reduced amount of water is added like say about 1 1/3 to 1 1/2 cup sufficient enough to cover most of the meat.

So please be wary of this, you can always turn the meat after various times and checking. Cheers

Rub your roast with oil, crushed garlic, sp, OR sprinkle with fav meat rub. Slice up onions into bottom of crockpot sit roast on it drizzle with little balsami vinegar cook on low. Make onion gravy with juices.