Heat small amount of olive oil in heavy frying pan, add mushrooms and saute several minutes, until barely starting to brown. Add to Crockpot. Add diced onion to same pan, adding more oil if needed, and cook about 5 minutes. Add to Crockpot. Again, add more oil if needed, then add diced beef and brown well, about 10-15 minutes. Don't rush the browning step. Add beef to Crockpot.
Add 1 cup beef stock to the pan and simmer a few minutes until slightly reduced, scraping off all browned bits. Add to Crockpot. Add diced tomatoes and juice, tomato sauce, balsamic vinegar, olives, garlic, rosemary, parsley, capers, and black pepper. Stir gently to combine, then put lid on Crockpot and cook 6-8 hours on low. (You could cook this 3-4 hours on low, but to for a recipe with large pieces of garlic like this I prefer the low setting.) Serve hot.
If you do all the advance preparation the night before and put the combined ingredient in the refrigerator overnight, this could cook as long as 10 hours in the Crockpot while you're at work. Manufacturers don't recommend putting crockery liner with prepared ingredients in the refrigerator; store in another container so contents stay cold, then put in Crockpot in the morning.