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Preheat oven to 425 degrees F.
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Rinse and pat fish dry. Cut into 2-inch sections. Sprinkle on both sides with sea salt.
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Mix the chili powder, cumin, garlic powder, black pepper, and cayenne (if using) in a wide shallow bowl. Press fish pieces into spice mixture on both sides and arrange on a baking sheet. Drizzle fish with olive oil.
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Bake 8 - 12 minutes, or until an instant read thermometer hits 145 degrees F.
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Meanwhile, set a heavy skillet over medium-high heat. Add the green onions and char all over, about 2 minutes a side. Slice off the root end and rough chop. In the pitcher of a blender, combine green onions, avocado, cilantro, lime juice, sea salt, and 3 tablespoons water. Add additional water, a tablespoon at a time, as needed to get the blending/texture right.
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Warm tortillas in the same pan used for the onions, 1 - 2 minutes per side.
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Layer tacos with green cabbage, fish, avocado sauce and chilies. Serve right away with lime wedges.