Start by whisking together minced garlic, chili powder, cumin, paprika, salt, black pepper, lime juice, and olive oil in a mixing bowl. Place your flank steak into a resealable bag or shallow dish, and pour the marinade over it. Seal it up and refrigerate for at least 30 minutes – the longer, the better for those rich flavors to develop!
Preheat your grill or grill pan over medium-high heat. Remove the steak from the marinade, letting any excess drip off. Place the steak on the grill, cooking 4-5 minutes per side for a medium-rare finish. Once done, set it aside to rest for 5 minutes. Thinly slice it against the grain to lock in tenderness.
Heat a tablespoon of vegetable oil in a skillet over medium-high heat. Toss in sliced bell peppers and onions, stirring occasionally. Cook until they’re tender and slightly caramelized, about 5-7 minutes. These add a sweet, smoky kick to your quesadillas!
Grab a tortilla and sprinkle shredded cheese over half of it. Top the cheese with some sliced steak, sautéed peppers, and onions, then add another layer of cheese for extra meltiness. Fold the tortilla in half, gently pressing down.
Place the folded tortilla in a heated skillet with a touch of oil. Cook for 2-3 minutes per side, until it turns golden brown and crispy. The cheese should be perfectly melted by the time you flip it! Repeat for all your tortillas.
Slice the cooked quesadillas into wedges and plate them up! Serve with sour cream, guacamole, and salsa on the side for dipping. Sprinkle some fresh cilantro for added color and flavor.