Place cut flank steak in a bowl.
Add 1 envelope fajita seasoning. Stir and toss to mix and cover steak well with seasoning.
Grill or sauté steak or chicken until done to your liking (See Steak Chart Below this Recipe). Set aside. (May be done in advance. Store in a tightly sealed container, in the refrigerator, until ready to use.)
Pour oil into a large skillet. Place skillet over medium-high heat.
When oil is hot and shimmering, add onions and peppers. Sauté 2 to 3 minutes, or until onions are just starting to get tender and fragrant (Do not overcook - onions and peppers will finish cooking in the over later). Set aside. (May be done in advance. Store in a tightly sealed container, in the refrigerator, until ready to use.)
Preheat oven to 350°F.
Cover a baking sheet with aluminum foil.
Arrange tortilla chips, in a single layer or small mound, on baking sheet.
Top chips with cooked flank steak.
Sprinkle with cheese.
Arrange prepared onion and bell pepper slices over cheese.
Sprinkle with remaining fajita seasoning, to taste (You do not need to use the entire envelope, a little goes a long way).
Place in preheated oven. Cook for 20 - 30 minutes, just until cheese has melted to your liking.
Remove from oven. Serve straight from baking sheet or, very carefully, use a large spatula or pizza peel, to move nachos to a serving platter.
Toss lettuce over chips.
Serve with pico de gallo, sour cream, guacamole and jalapeño slices in individual serving bowls or dolloped and scattered over top or around chips.