Preparation

Make the salsa

• Combine the orange and lime juices, onion, chile, oregano, and salt in a medium bowl. Cover and refrigerate until the onion begins to turn pink, about 2 hours. Stir in the mango.

Marinate the fish

• Heat an 8-inch skillet over medium-low heat. Toast the cumin seeds, peppercorns, and allspice berries in the skillet, shaking occasionally, until fragrant and the cumin seeds are a shade darker, about 2 minutes. Transfer to a spice grinder or mortar. Add the annatto seeds, oregano, and cayenne and grind to a powder. Transfer to a small bowl.
• Mash the garlic with 1/2 tsp. salt into a paste with a mortar and pestle or the side of a chef’s knife. Add to the spices along with the orange and lime juices and olive oil. Stir until well combined. Rub the marinade all over the fish. Transfer to a plate and refrigerate, covered, for at least 30 minutes and up to 2 hours.

Assemble the tacos

• Prepare a medium (350°F to 375°F) gas or charcoal grill fire for direct grilling. Oil the grill grate. Working in batches, heat the tortillas in a single layer, turning once, until soft with light grill marks. Stack in a cloth-lined basket or plate and wrap to keep warm.
• Brush both sides of the fish with olive oil and lightly season with salt. Grill, turning once or twice until just cooked through (the tip of a knife inserted into the center should be warm when you touch it against your lip), 8 to 12 minutes total.
• Transfer the fish to a cutting board. Coarsely shred the fish with two forks. Distribute among the tortillas, and use a slotted spoon to top each with salsa. Garnish with the avocado slices and serve.

Make Ahead Tips

The salsa will keep, refrigerated, for 1 day. Bring to room temperature before serving.