Halibut is easy to grill and lends itself to almost any flavor. The achiote and pineapple work beautifully together to create that great summer taste.
Light a charcoal fire and let the coals burn until they are covered with gray ash. Position the grill grate and let it heat for a couple of minutes.
When the grill is hot, brush each slice of onion and pineapple with oil. Grill the slices until nicely grill-marked on each side, about 2 to 3 minutes total, 5 to 7 minutes for the onion. Remove to a platter.
Put achiote paste and lime juice into a blender and process until smooth.
Season fish with salt, then spray lightly with oil. Place on the grill. Cook without turning until fish is nicely grill-marked on bottom, 3 to 4 minutes. Carefully flip fish onto a cooler portion of the grill. Continue grilling until fish gives slightly under the firm pressure of a finger, about 3 minutes more. Transfer to the platter with the pineapple and onion.
Garnish with the lime wedges. Serve with the salsa.
Put fish into a baking dish and smear on both sides with the achiote mixture, using about 2 tablespoons total. (Save remaining achiote mixture for another meal. It will keep in the refrigerator for several days.)