Peel and slice the onion. Layer half of the onion slices in the bottom of your slow cooker insert.
Microwave the 4 tablespoons of butter in a 4-cup glass measuring cup until melted, about 1 minute.
Whisk the 4 tablespoons of flour into the butter and microwave for one additional minute. Whisk again. Repeat two more times, microwave and then whisk, to make the roux.
Into the prepared roux, whisk in the beef broth until smooth. Add the crushed tomatoes, Worcestershire sauce, Italian seasoning, garlic, salt and pepper and whisk again. Stir in the (undrained) mushroom stems and pieces to the gravy.
Place two of the cube steaks on top of onions. Top with half of the mushroom gravy.
Repeat the layers: add the remaining sliced onions to the Crock-Pot insert. Top with the remaining two cube steaks. And smother the steaks with the remaining half of the mushroom gravy.
Cook on LOW heat for 6 - 8 hours, or until the steak is tender and the gravy is thickened. Serve over mashed potatoes.