I got this recipe from The Food Nanny cookbook and it's great. I really like cooking sirloin roasts in the crockpot. They aren't a fatty as classic pot roasts, but they have enough marbling to make the roast moist after the slow and low cooking. Also the sauce you end up with at the end is so yummy over the roast. This recipe is a keeper!
Liz's Crockpot Roast Beef
4 lb beef sirloin roast
1/3 cup brown sugar
3 tablespoons cider vinegar
1 tablespoon soy sauce
1 tablespoon Worcestershire sauce
1 can cream of mushroom soup
1 packet onion soup mix
1 can low sodium beef or chicken broth
5 carrots peeled and cut into 1" pieces
6 red potatoes, scrubbed and quartered
Place roast in crockpot (add vegetables too if using). In a bowl mix together: brown sugar, cider vinegar, soy sauce, worcestershire, soup, onion soup mix, and broth. Cook on low for 12 hours. Alternatively you can cook on high for 5 hours, then turn to low for 3 hours. Remove roast and break into serving size pieces. Strain juices from crockpot and serve over roast.