Preheat oven to 325°F. Place the tomatillos, onion and garlic on a baking pan. Drizzle with the olive oil and cover with foil. Bake until tender (about 30 mins.). Cool to room temperature.
Transfer the cooked vegetables to a food processor. Add the cilantro, serrano pepper and lime juice. Cover and pulse until chunky. Season with salt and pepper. Refrigerate until serving.
For the Tacos:
In a large plastic bag, add the lime juice, oil, salt, and Mexican seasoning mix and shake to combine. Add halibut. Seal the bag and turn to coat the fish evenly. Refrigerate for up to 1 hour.
Drain and discard marinade. Grill halibut, with the BBQ lid covered, over medium heat for 4-6 minutes on each side or until the fish flakes easily with a fork. Keep an eye on it as it cooks quickly. Flake fish into 1 ½” pieces. Season again, if needed.
Warm the tortillas by heating the grill to medium-low. Grill them until warm, less than 1 minute per side. You can keep them warm on a plate, covered with a clean kitchen towel.
To serve, top tortillas with cabbage, fish, diced mango and tomatillo salsa. Garnish with red chili peppers, lime wedges and more fresh cilantro.