Wrap stacked tortillas in aluminum foil, and heat in oven 15 minutes or until hot. To microwave, wrap a stack of tortillas lightly in paper towels and warm on high for 6 or 7 seconds per tortilla.
In a large frying pan over medium-high heat, brown beef; drain, leaving beef in the pan.
Add onion, chile or enchilada sauce, salt, and pepper; cook 2 to 3 minutes or until onions are transparent.
In the center of each warm tortilla, spoon 1/6 of meat filling. Fold tortilla, tucking in the ends to make a secure fat tube, and fasten with wooden toothpicks. NOTE: Assemble only 2 or 3 at a time, as the tortilla will absorb liquid from the sauce.
In a large pot or deep fryer over medium heat, add at least 4 inches of vegetable oil. Heat oil to 375 - 400°F Fry the chimichangas, 1 or 2 at a time, 3 minutes or until golden brown. Remove from oil, drain and place on serving plates.
Top with Cheddar cheese, lettuce, green onions, and any optional ingredients that you prefer.