Spray slow cooker with non-stick cooking spray. Season roast with salt and pepper. Place meat in pot with fat side up. Pour tomato sauce over roast. Place onion rings over all. Toss in bay leaves. Cover and cook 1 hour on high.

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

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