In medium saucepan pour beef stock, bring to boil, keep boiling until reduced to about 1/2 cup guessing at 3 minutes as it really depends upon pan size and burner temperature. Add onion and garlic.
Meanwhile, mix together black pepper, onion powder, and garlic powder; rub onto roast.
Sprinkle flour onto both sides of meat and pat into meat-recently saw that technique is called SPANKING the meat-lol!
Heat olive oil in high sided pan at moderate temperature, sear meat all over for about 8 minutes-take your time as this is when you get luscious drippings.
While meat is browning, line crockpot.
Put meat into crockpot, deglaze pan (about 1 minute) that meat was in with 1/4 cup water, scrape up those yummy drippings, reserve in pan for next step.
Pour reduced beef stock with onion and garlic, balsamic vinegar, and rosemary into deglazed liquid; pour over meat.
Cover crockpot, cook on low. I check the meat with meat thermometer about 30 minutes before I think it will be done. Crockpots vary. For 3.5 pound roast, it will be done in about 5 hours, and for 3.25 pound roast it will take about 4 hours and 50 minutes. After removing meat, cover with foil and let rest while making gravy.
If using shitake-About 15 minutes before removing roast, soak mushrooms in boiling water for 30 minutes, drain and discard liquid.
Take liquid with onion from crockpot, and pour into large skillet, add the other cup beef stock, bring to boil, boil about 5 minutes, make slurry of flour and cold water, reduce burner to moderate temperature, stir slurry into hot liquid, and stir until lumps are out, strain if needed. Taste for salt.
Put reserved mushrooms into gravy.
Slice meat thin and add to gravy.
If you are like me and make this ahead, I take it out of fridge about an hour beforehand, let it set at least 30 minutes to get to room temperature, then I add more beef stock to it as reheat it and am guessing at amount as I have not measured it.