In a bowl, combine the melted Hatch Chile butter, lime juice, salt, and pepper.
Place the flank steak in a large resealable plastic bag or a shallow dish. Pour the marinade over the steak, ensuring it is well-coated. Marinate in the refrigerator for at least 30 minutes.
Preheat a grill or grill pan over medium-high heat.
Remove the steak from the marinade and let any excess drip off.
Grill the steak for about 4-5 minutes per side, or until it reaches your desired level of doneness.
Let the steak rest for 5 minutes before slicing it thinly against the grain.
While the steak is grilling, heat the olive oil in a large skillet over medium-high heat.
Add the sliced bell peppers and red onion to the skillet. Sauté until the vegetables are tender and slightly charred, about 7-8 minutes.
Warm the tortillas in a dry skillet or microwave.
Assemble the fajitas by placing a few slices of steak on each tortilla. Top with the sautéed bell peppers and onions, and sprinkle with chopped cilantro.
Serve the fajitas with lime wedges on the side for an extra burst of flavor.