Preheat oven to 375°F with the top rack positioned in the middle. Line a baking sheet with parchment or foil (this is only for easy cleanup).
In a small dish, combine the olive oil and taco seasoning. Chop the veggies. Cut the cauliflower florets into small pieces, about 1 ½ inches. If they're thick, slice them in half.
Spread the cauliflower florets, onions, and bell peppers on a sheet pan. Drizzle the seasoning mix over the top and toss to coat all the veggies.
Roast:
Cook the veggies in the preheated oven for 30 minutes, until the tops of the cauliflower florets turn golden brown, and the onions and bell peppers are tender. Open the oven after 20 minutes and toss them around. Remove them from the oven.
Serve:
Warm your tortillas, fill them with the roasted veggies, and top them with your favorite toppings like salsa, sour cream, or cilantro sauce.