Preheat the oven to 400°F.: The kitchen fills with a faint warm scent as you preheat, which signals the start of roasting. A properly preheated oven ensures the fish cooks quickly and develops an even, opaque texture without drying. If the oven is not fully heated, the fish may steam and stay translucent in places, so always allow it to come to temperature before you slide the sheet in.
In a small bowl, mix the taco seasonings until well combined. Set aside 1/2 teaspoon of the taco seasoning for the coleslaw.: The aroma of chili powder, paprika, and cumin will become more pronounced as you stir them together, which helps you sense balance. Reserving a pinch for the slaw ties the flavors together. A common mistake is overmixing with wet ingredients at this stage, which can turn the spices pasty; keep them dry until you need them.
Cut the fish filet into 1-inch pieces and add to a large bowl. Drizzle with the oil and taco seasonings and toss until well coated. Line a baking sheet with foil and spray with cooking spray. Scatter the fish pieces on the baking sheet and bake for 10 minutes or until the fish is opaque. Raise the heat to BROIL and cook for 3 more minutes to crisp the outside.: When you cut the fish and toss it, you will feel the pieces become slick with oil and spice. As it roasts, the kitchen will fill with a toasty, seasoned scent and the flesh will turn from translucent to a chalky opaque, which is the visual cue that it is nearly done. Broiling briefly encourages the edges to caramelize and crisp, creating a satisfying texture contrast. Avoid crowding the pan, which causes steaming rather than roasting, and watch closely under the broiler to prevent burning.
Add the coleslaw mix, green onion, and cilantro leaves to a medium bowl. In a blender or food processor, blend the avocado, sour cream, lime juice, adobo sauce, taco seasoning, and water until smooth. Toss half of the avocado sauce with the coleslaw mix.: As you blend the avocado with lime and sour cream, you will notice a creamy, fragrant emulsion form that smells bright and slightly smoky from the adobo. Tossing the slaw with half the sauce softens the cabbage just enough to meld flavors while retaining crispness. A frequent error is using an underripe avocado, which leads to a stringy or bland sauce; choose a ripe one that yields to gentle pressure.
Divide the rice, coleslaw mix, and fish among four bowls and serve with lime wedges, sliced avocado, more chopped cilantro, pickled red onions, and/or pico de gallo. Drizzle with the reserved sauce as desired.: When plating, contrast is your friend visually and texturally. Spoon warm cilantro lime rice as a base so steam lifts the aromas, add the dressed slaw for crunch, and top with the warm, flaky fish . Garnishes like pickled onions and pico de gallo introduce bright notes and a pop of color. Let guests add the reserved sauce to control creaminess. Avoid assembling too far in advance, which can lead to sogginess; instead serve soon after plating for the best texture.
