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.
