Line a large, rimmed baking sheet (for best results, use a three-quarter sized sheet pan) with a silicone baking mat (recommended) or parchment paper. Preheat oven to 400°F.
Combine flank steak, peppers, red onion, olive oil, cumin, chili powder, cayenne, and red pepper flakes in a large bowl. Season generously with salt and pepper to taste. Mix until everything is well coated.
Spread the pepper and steak mixture onto the baking sheet. Make sure everything is in an even layer with little to no overlap. Roast in the oven for 15-20 minutes, or until the peppers have softened and slightly caramelized and the steak reaches the desired doneness level.
Remove sheet pan from oven. Sprinkle with fresh cilantro and a squeeze of lime juice.
Serve immediately with avocado (optional) and either corn tortillas or lettuce wraps.
The nutrition facts have been calculated without the optional avocado and corn tortillas. If you eat your sheet pan fajitas with one or both optional items, keep that in mind when you look at the nutrition facts.