Heat oil in a large skillet over medium heat. Stir in the onion and garlic, and cook until tender and transparent, about 5 minutes.
Mix in the roast beef, taco sauce, diced green chile peppers, cumin, and red pepper flakes.
Bring the mixture to a boil. Reduce heat to medium, and simmer, uncovered, for 5-10 minutes, or until sauce reduces and thickens.
Arrange warmed 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, tomatoes, lettuce and a dollop of sour cream.
Add green onions, and sliced olives if using.
Fold over ends and roll up sides to make a burrito, encasing the filling.
Serve with salsa and additional sour cream.