1 tablespoon vegetable oil
1 onion, chopped
1 clove garlic, minced
4 tomatoes, chopped
2 cups chopped cooked roast beef
1 (8 ounce) jar prepared taco sauce
1 (4 ounce) can diced green chile peppers
1/2 teaspoon cumin
1/8 teaspoon red pepper flakes, or to taste (optional)
6 (7 inch) flour tortillas, warmed
1 1/2 cups shredded Cheddar cheese
2 cups shredded lettuce
Heat the oil in a skillet over medium-high heat. Stir in the onion and garlic, and cook until tender and transparent, about 5 minutes. Mix in the tomatoes, roast beef, taco sauce, chile peppers, cumin, and red pepper flakes, if desired. Bring the mixture to a boil. Reduce heat to medium, and simmer, uncovered, for 25 minutes, or until thickened.
Arrange tortillas on a clean, flat work surface. Spread about 2/3 cup of beef mixture in the center of each warm tortilla. Sprinkle evenly with cheese and lettuce. Fold over ends and sides to make a package.