Cut slits in the roast and insert garlic and tomatoes into each. I found this went faster when I made larger slits and stuffed in a bunch or used the fat marbling and got up under it like getting spices under chicken skin. Whatever works, but prepare both sides of the meat this way.
Rub the roast in salt and pepper.
Arrange the onion slices in the bottom of the crock pot and put the roast on top.