Pat the flank steak dry and lightly score both sides without cutting too deeply. Season generously with salt.
Combine the avocado oil, lime juice, honey, chili flakes, cumin, serrano pepper, garlic, and cilantro in a bowl.
Place the flank steak in a large zip-top bag or container. Pour the marinade over the steak, seal, and refrigerate for at least 4 hours or up to 24 hours.
About 1 hour before grilling, prepare the bell peppers and onion and heat the grill for direct cooking.
Place a large cast-iron skillet on the grill. Add a little avocado oil, followed by the bell peppers and onion. Season with salt and sauté for 4–6 minutes.
Place the steak directly over the heat and sear for 2 minutes. Rotate it 90 degrees, then flip and repeat on the other side. Cook until the internal temperature reaches about 125°F.
After flipping the steak, add the honey and balsamic glaze to the peppers and onions. Toss and continue cooking until caramelized.
Move the skillet to a low-heat area of the grill to keep the vegetables warm.
Rest the steak for 8–10 minutes, then slice it.
Add the sliced steak to the skillet with the vegetables and toss together.
Toast the flour tortillas on the grill.
Fill each tortilla with steak and caramelized vegetables. Finish with cilantro and a squeeze of fresh lime juice.