Mix flour and mustard. Dredge steak in flour mixture and pound in with a cleated meat mallet. Cut pounded steak into serving size pieces.
Heat oil in heavy frying pan over medium-high heat. Add meat and brown on both sides. Transfer to crock pot.
Add tomatoes with liquid, onions, celery, carrots, Worcestershire and sugar to meat. Season to taste with salt and pepper. Cover and cook on high for about 3 to 4 hours or until meat is tender. Serve with rice or potatoes and your favorite veggies.
NOTE: You may also do this in the oven. Once meat has been browned, place in a shallow baking dish, add tomatoes and remaining ingredients, salt & pepper to taste. Cover and bake at 350 degrees for about 1 1/2 to 2 hours or until meat is tender when pierced.