Preheat the Oven: Place the rack in the center of the oven and preheat to 400°F (200°C). Line a baking sheet with foil.
Prepare the Spice Mixture: In a small bowl, mix the cumin, paprika, garlic salt, salt, and black pepper.
Roast the Vegetables: On the prepared baking sheet, toss sliced bell peppers and onion with 1 tablespoon olive oil and ¼ of the spice mix. Bake for 5–7 minutes, until tender and lightly browned. (If the vegetables are thinly sliced, you can skip pre-baking and add them with the fish.)
Add the Fish: Place the fish fillets beside the vegetables, drizzle with the remaining oil, and evenly coat with the remaining spice mix. Ensure everything is arranged in a single layer without overlapping. Return to the oven and bake for 15 minutes, or until the fish is opaque and flakes easily with a fork.
Prepare the Avocado Sauce: While the fish and vegetables are baking, make the sauce. Cut the avocado in half, remove the pit, and scoop the flesh into a blender or food processor. Add the cilantro, onion, and serrano pepper (if using). Squeeze in the lime juice and season with salt to taste. Blend until smooth or to your desired texture.
Assemble the Tacos: Once the fish and vegetables are done, remove them from the oven. Use forks to shred the fish and set aside. Warm the tortillas. Spread a bit of the avocado sauce on each tortilla. Add the shredded fish, roasted peppers, cabbage, and fresh cilantro. Finish with a squeeze of lime juice.
Serve & Enjoy!