Apply non-stick cooking spray to the slow cooker pot. Rub the meat with salt and pepper and put it in the pot fatty side up. Cover the meat with tomato sauce and onion half-moons on all sides. Add bay leaves. Turn the slow cooker on high and cook under the lid for 1 hour.
Reduce heat to low and leave to cook for 6 to 8 hours. Take the meat out of the pot and transfer it onto a warm platter.
Strain the drippings through a fine sieve into a medium saucepan. Whisk in 3 tablespoons of flour and cook over medium heat until thickened, stir non-stop! Season with more salt and pepper, if desired. Serve the roast with the sauce on the side.