Preheat oven to 400°F (200°C).
Line a large sheet pan with parchment paper.
Add the sliced onions and sliced peppers.
Drizzle with olive oil and fajita seasoning.
If using chicken or steak, season with fajita seasoning and add to sheet pan (optional)
Bake in the preheated oven for 25-35 minutes, or until the vegetables are tender and slightly caramelized. Check that the meat is fully cooked before removing.
Place a corn tortilla on a parchment lined baking sheet.
Top with a generous amount of the cooked vegetables and protein (if using).
Sprinkle with shredded cheese.
Fold the tortilla in half and place it on a baking sheet. Repeat with the remaining tortillas. Pro tip: If tortillas are dry and risk cracking, add to oven for 1-2 minutes, then remove and fold in half. After the 1 minutes they will be softer and easier to fold.
Bake for 5-8 minutes or until the cheese is melted and bubbly.