Combine flour, mustard, salt, and pepper. Coat meat on both sides with flour mixture.
Heat 2 tablespoons of oil in a heavy skillet. When oil is hot, brown meat on both sides.
Place browned steak in a slow cooker or large roaster.
Add another tablespoon of oil into the now-empty skillet. Add chopped onions to the skillet and cook until softened--about 3 minutes
Add 1 cup of finely chopped carrots to the onions and brown for a couple more minutes. Combine brown sugar, Worcestershire sauce, and tomatoes with the carrots and onions. Stir together and pour over meat.
If using a slow cooker, set to low for 6-9 hours. Or bake in an oven at 325˚F for 2 1/2 to 3 hours.
When the meat is fork-tender, remove it from the pot and set aside on a serving platter to rest. Cover with foil.
Gravy in the microwave:
Skim as much fat as possible off the top of remaining pot juices.
In a large glass bowl or Pyrex 2-qt pitcher, combine cold water and cornstarch. Add pan juices to the cornstarch mixture. Add water if necessary for a total of 2 1/4 c. liquid.
Mix well with a whisk. Microwave on HIGH for 3 minutes and stir well. Microwave for 3 more minutes or until mixture thickens and boils for 1 minute. Whisk again. Add salt and pepper to taste.
Alternate instructions for gravy on the stove
Skim as much fat as possible off the top of remaining pot juices.
Combine cornstarch and 1/4 cup of cold water. Add to the pot juices and whisk until smooth.
Cook the gravy over medium-low heat until it thickens. Add salt and pepper as desired.