Preheat the oven to 400° F. Prepare two baking sheets with parchment paper.
Heat the olive oil in a large skillet over medium heat. When the oil shimmers, add the onion. Cook, stirring occasionally, until the onion is translucent, about 2 minutes.
Add the garlic and cook for 1 minute.
Add the chipotle peppers, tomato paste, broth, cumin and chilli powder. Let the sauce simmer until the sauce has thickened slightly, about 5 minutes. (Add more liquid if you want a bit more sauce.) Remove from the heat and pour the sauce over the cauliflower pieces in a bowl and stir to coat.
Spread cauliflower out onto one of the baking sheets in a single layer. Bake in the oven for about 18-20 minutes.
Remove from the oven and fill each tortilla with a spoonful of cauliflower and cheese, fold and place on the other prepared baking sheet.
Bake in the oven for 10 - 12 minutes, or until cheese is melted and the tortillas are golden brown. Serve tacos with lettuce, sour cream and salsa.
