Prepare Pickled Onions up to 1 day ahead. Cover and refrigerate until ready to serve. Prepare Salsa Roja, and set aside.
Lightly brush halibut fillets on both sides with oil. Season evenly with salt and pepper to taste. Grill, covered with grill lid, over high heat 8 to 10 minutes or until fish is barely opaque in the center, turning once.
Transfer to a medium bowl, and break into pieces with a fork.
Heat tortillas on the grill 20 to 30 seconds per side. Arrange fish in the center of each tortilla, and top with onions and salsa. Wrap loosely, and serve warm.
