Place cauliflower florets in a large bowl and drizzle on 1 tablespoon olive oil. Toss to coat. Mix together the chile powder, paprika, cumin, salt, and garlic powder, and sprinkle it over the cauliflower. Toss again to coat evenly with the spices.
Spread cauliflower onto a rimmed baking sheet in an even layer. Place the bell pepper and onion slices among the cauliflower. Drizzle the remaining olive oil over the whole pan.
Place in the oven and roast for 30 minutes, until cauliflower is tender and starting to caramelize on the bottom, stirring halfway though if desired.
Serve on warmed tortillas with crumbled cotija, avocado, and your favorite toppings.