Preheat the oven to 400°F.

Line a large baking sheet with a silicone baking mat or parchment paper. Set aside.

Spray a 9x13-inch baking dish with nonstick cooking spray. Set aside.

Using a cheese grater or food processor fitted with a grating attachment, grate the Cheddar and Gruyere cheeses. Set aside.

Transfer the grated cheese to a large bowl, and add the Parmesan cheese. Set aside.

Cut the cauliflower into 1-inch florets.

Arrange the cauliflower florets on the prepared baking sheet. Drizzle with olive oil and season with salt and pepper. Stir to distribute the seasoning.

Roast the cauliflower on middle rack of oven until it can be pierced with a fork, but is not tender, about 20 minutes.

When cauliflower is done, remove from oven and set aside.

Reduce the oven temperature to 350°F.

In a large pot over medium heat, melt the butter.

Once melted, add the flour and whisk to thoroughly combine. Continue whisking until the roux turns slightly golden and begins to smell toasty, about 1 minute.

Slowly stream the milk into the pot while continuing to whisk. Cook the mixture, whisking steadily, until it begins to thicken and bubble, about 4 minutes.

Once mixture thickens and bubbles, turn off the heat. Add the mustard powder, garlic powder, salt, and black pepper.

Add 3/4 of the cheese to the pot one handful at a time, whisking slowly after each addition. Reserve a generous 1 cup for topping.

Once the sauce is smooth, add the roasted cauliflower to the pot. Stir until combined.

Pour the contents of the pot into the prepared baking dish. Sprinkle the reserved cheese in an even layer over the top of the cauliflower.

Bake cauliflower mac on middle rack of oven until the edges are bubbling and the top is lightly browned, 15-18 minutes

Check to see that cauliflower mac is done. Remove from oven or add time as needed.

Allow dish to rest for about 15 minutes before serving.