Preheat oven to 400 degrees. In a large bowl, toss cauliflower florets with two tablespoons olive oil, garlic powder, salt, and pepper. Spread on a baking sheet and roast until brown and softened, about 15 to 20 minutes. Set aside to cool then place in an airtight container.
In a small saucepan over medium heat, warm two more tablespoons of olive oil. Add black beans and cumin and season with salt and pepper. Cook, stirring frequently until beans are warmed through, about two to four minutes. Remove from heat and add lime juice, then coarsely mash the beans using a fork or spoon. Set aside.
To serve, warm each tortilla in the microwave for 20 seconds. Top with mashed black beans, cauliflower, avocado, and microgreens. Squeeze lime wedges over top.