Cut the head of cauliflower in half, remove the stem, and chop into small pieces. Set aside.
Heat the oil in a large cast-iron skillet over medium heat. Add the garlic and onion and cook until fragrant, about 2 minutes.
Add the corn, carrots, black beans, and 2 cups (600 g) of cauliflower and cook, stirring often, for another 3 minutes, until the corn starts to brown around the edges.
Add the flour, chili powder, and salt and stir until the vegetables are evenly coated.
Add the milk and diced tomatoes and cook, stirring constantly, until thickened, about 3 minutes.
Add the mozzarella cheese and stir until melted.
Turn off the heat and arrange the tater tots on top of the vegetable mixture.
Transfer to the oven and bake until browned and bubbling, about 30 minutes.
Garnish the casserole with tomato, red onion, jalapeño, and cilantro, if using.