Begin by trimming as much visible fat as you can from the roast.
Rub meat well with steak seasoning.
Heat a Dutch oven or heavy pan with the olive oil on medium-high and brown roast well on all sides. This will take a few minutes but be patient and don't skip this step!
While roast browns put 1 cup beef stock in saucepan, bring to a boil and cook until reduced to 1/2 cup, then let cool slightly.
Mix in balsamic vinegar and pureed tomatoes.
Place onion slices in the bottom of the crockpot. Place browned roast on top of onion slices.
This is another step not to skip. Deglaze the pan you browned the roast in with 1/4 cup water and then add to tomato-balsamic sauce mixture.
Pour the sauce mixture over the roast and onions.
Cook on low for cook 6-8 hours, until beef is tender. If desired, you can turn the roast over a couple of times during the cooking time but it is not necessary. I do it if I am home but otherwise I don't.
About 30 minutes before the roast is finished, you can opt to thicken the juices in the crockpot by removing about 1/2 - 1 cup of the juices and thicken with a tablespoon of cornstarch. Whisk well to remove any lumps. Pour back into crockpot, give a stir and let thicken for about 30 more minutes. Serve with the sauce and onions over the roast.
If you opt to not thicken the juices, you can serve the roast as is with the juices and onions in the crockpot drizzled over the roast.
When making this roast, I'll serve it with Mashed Potatoes and roasted vegetables.