Season the flank steak evenly with sea salt, black pepper, and ground cumin.
Heat avocado oil in a large cast-iron skillet over medium-high heat until shimmering.
Place the steak in the skillet and sear for 3 to 4 minutes per side for medium-rare.
Remove the steak from the pan and let it rest on a cutting board for 5 minutes.
In the same skillet, sauté the sliced bell peppers and red onion until tender and slightly charred.
Slice the rested steak against the grain into thin, bite-sized strips.
Wipe the skillet clean, then place the corn tortillas inside over medium heat.
Layer the cheese, steak strips, and sautéed vegetables on one half of each tortilla.
Fold the tortillas over and cook for 2 minutes per side until the cheese is melted and the exterior is crispy.
Garnish with fresh cilantro before serving.