Preheat your oven to 425 degrees f.
Line a large baking sheet with parchment paper and assemble your fish filets on the baking sheet.
Gently pat your fish filets dry using a paper towel.
In a small bowl combine all the seasonings together with your oil and melted butter. Drizzle this mixture equally over your fish and rub it on all sides.
Roast the fish in your preheated oven for 18-20 minutes, or until cooked through and flaky. Timing will vary depending on the thickness of your fish.
Meanwhile, prepare your toppings for the tacos. Thinly slice the purple cabbage, dice avocado, and thinly slice green onions; Quarter the cherry tomatoes and chop a handful of fresh cilantro.
Add all of your taco sauce ingredients to a small bowl and whisk until really well combined.
Once the fish is almost cooked through, carefully place the tortillas on top to heat up for about 1 minute. Alternatively you can heat up your tortillas in a skillet on the stove.
Assemble your fish tacos starting with a warm tortilla.
Add in a generous piece of fish, then fill with the prepared toppings as desired.
Drizzle with your taco sauce and serve immediately.