Place a large piece of aluminum foil on a baking sheet.

Arrange the mahi mahi fillets side by side on the foil.

Sprinkle the fish with 1/2 teaspoon of kosher salt.

In a blender, combine achiote paste, lime juice, orange juice, oregano, cumin, scallions, chile powder, vinegar, and brown sugar.

Pulse until the mixture is smooth.

Pour the mixture over the fish fillets, using tongs to coat evenly.

Scatter thyme sprigs and bay leaves over the fish.

Top with orange slices.

Cover the fish with another piece of foil.

Crimp the edges of the foil to tightly seal.

Marinate the fish in the refrigerator for 1 to 4 hours.

Position a rack in the upper third of the oven and preheat to 475 degrees F.

Transfer the fish to the oven and bake until tender, approximately 25 minutes.

Remove the herbs and orange slices from the baked fish.

Prepare warmed corn tortillas.

Serve the fish in tortillas with desired toppings.

To make charred spring onions, trim 2 bunches spring onions, halving lengthwise if large.

Cook spring onions in a large cast-iron skillet over medium-high heat until tender and blackened, 5 to 8 minutes.

Cut the spring onions into pieces and sprinkle with lime juice and salt.