Roast Beef Tacos

1 tsp salad oil
1 1/2 lb. cooked, shredded roast beef (pressure cooked or crockpot cooked)
1/2 small onion, chopped
1/2 tsp garlic powder
1/4 tsp salt
pepper (coarsely ground) as desired
3 to 4 chopped stewed tomatoes
pinch of oregano
8 to 10 ready-made taco shells
thinly-shredded lettuce
shredded cheese

In a skillet, heat oil and briskly saute roast beef. Break up strands of meat. Add onion and seasonings. Add tomatoes and oregano, heat until well blended. Cover and let simmer. Warm taco shells. Fill taco shells with roast beef. Garnish with lettuce and shredded cheese. Serve with refried beans or rice!