Heat the oil in a large, non-stick skillet set over medium-high heat. Add the onion and bell peppers and cook, stirring, for 2 minutes. Mix in the cumin, chili powder, salt and hot pepper sauce. Toss in the prawns and cook 2 to 3 minutes more, or until the prawns are just heated through. Spoon the mixture into a serving bowl and place on the table with the cheese, lettuce, salsa and tortillas in appropriate vessels alongside. Allow diners to stuff and roll their own fajitas.