Spray slow cooker with nonstick cooking spray. Season roast with salt and pepper. Place meat in the pot, fat side up. Pour tomato sauce over roast. Place onion over top. Toss in bay leaves. Cover and cook for 1 hour on High.

After 1 hour, reduce heat to Low and cook 6 to 8 more hours. Carefully lift meat out of the pot and remove to a warm platter.

Pour drippings through a strainer into a medium-sized saucepan and discard material in the strainer. Whisk flour into the liquid. Cook, stirring constantly over medium heat until thickened. Season to taste with salt and pepper; serve alongside roast.