Preheat oven to 400°F and line a large, rimmed baking sheet with parchment paper or a Silpat® baking mat. Set aside.
In a large mixing bowl, combine sliced flank steak, peppers, red onion, olive oil, cumin, chili powder, cayenne, and red pepper flakes. Generously season with salt and pepper, to taste, and mix until everything is well coated.
Spread the steak and pepper mixture onto the prepared baking sheet and roast in pre-heated oven for 15-20 minutes, or until the steak reaches the desired level of doneness and the peppers are soft and slightly caramelized.
Remove sheet pan from oven and sprinkle with fresh cilantro and a squeeze of fresh lime juice. Serve immediately with diced avocado, corn tortillas, or lettuce wraps for a low-carb option, if desired. Enjoy!