Light a charcoal fire and let the coals burn until covered with gray ash but still very hot; bank the coals to one side and set the grill grate in place. Or, heat one side of a gas grill to high.
Make the marinade In a small (7- to 8-inch) non-stick skillet over medium-high heat, roast the tomatillos (start them cut-side down), garlic and habanero chiles until soft and browned, 3 or 4 minutes per side. (If a non-stick skillet is unavailable, line a regular skillet with aluminum foil.) Cool until handleable.
Slip the garlic out of its skin, remove the stems from the chiles and place both into a blender. Add the lime and orange juices and salt, then blend until the mixture is smooth. Brush half the marinade over the filets.
Add the roasted tomatillo and fresh herbs to the blender jar and blend with remaining marinade until smooth. Pour the salsa into a small bowl and mix in the avocado and red onion.
Finishing the dish Brush or spray the filets with oil and place skin side up on the hottest part of the grill, cooking about 3 minutes per side. Remove from grill and serve with salsa.