Stir lime juice, olive oil, cilantro, jalapeno, chile powder, cumin, salt, and pepper together in a large bowl or resealable zip-top bag. Add halibut and marinate for 20 to 25 minutes. Do not over-marinate, as lime juice will start to 'cook' the fish.
Preheat an outdoor grill for medium heat and lightly oil the grate. Drain marinade; grill fillets for 5 minutes. Turn and cook until fish flakes easily with a fork, about 2 minutes more.
Warm tortillas on the grill or stove. Divide halibut among tortillas and top with cabbage, salsa, pepper Jack cheese, and avocado.
