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.
nutrition information (per serving):
17, Fat Calories
150, Saturated Fat
26, Monounsaturated Fat
49, Polyunsaturated Fat
Photo: Scott Phillips