In a small bowl, whisk together the chili powder, cumin, garlic powder, sea salt, black pepper, and lime juice to create a spice rub.
Rub the spice mixture evenly onto both sides of the flank steak and let it marinate at room temperature for 15 minutes.
Heat a large cast-iron skillet or grill pan over medium-high heat and add the olive oil.
Add the sliced red bell pepper, green bell pepper, and yellow onion to the skillet, sautéing for 5-7 minutes until they are tender-crisp and show slight char marks.
Remove the sautéed vegetables from the pan and set them aside on a warm plate.
Place the marinated steak in the same hot skillet and sear for approximately 4-5 minutes per side for medium-rare doneness.
Transfer the grilled steak to a cutting board and allow it to rest for 5 minutes to lock in the juices before slicing thinly against the grain.
Arrange the sliced steak and charred vegetables on a platter, then garnish with fresh avocado slices and chopped cilantro before serving.