Arrange a rack in the top third of the oven and heat to 400°F. Coat a 9x13-inch baking dish with cooking spray or butter.

Make the cauliflower cheese sauce. Bring a large pot of water to a boil over medium-high heat. Add the garlic, thyme and bay leaves and boil for 5 minutes to infuse water.

Add the cauliflower and cook until very tender, 7 to 10 minutes. Using a slotted spoon, transfer the cauliflower to a rimmed baking sheet or medium bowl out of the water, leaving the herbs and garlic behind in the pot (do not drain the pot).

Place 1/2 of the cauliflower in a blender or food processor fitted with the blade attachment. Scoop out 3/4 cup of the cooking liquid and add it to the blender along with 1 1/2 tablespoons of oil. Blend until very smooth, about 1 1/2 minutes. Transfer the puree to a large bowl. Repeat with the remaining cauliflower, 3/4 cup of cooking liquid and remaining 1 1/2 tablespoons of oil; transfer to the same bowl.

Add the cheddar, Gruye?re, mustard and pepper into the puree and whisk to combine. Taste and season with more pepper as needed; set aside.

For the macaroni and cheese: Bring the pot of infused water back to a boil. Add the pasta and cook per package instructions until just al dente.

Drain the pasta, discard the herbs and garlic cloves and add the pasta to the bowl of cauliflower sauce. Stir gently until the pasta is entirely coated.

Transfer the mixture to the baking dish and spread into an even layer. Sprinkle the butter evenly over the top, followed by the grated parmesan.

Transfer the dish to the oven and bake until the sauce is bubbling and the top is golden-brown, 15 to 20 minutes. If the pasta becomes bubbly before the top is golden, you can run the dish under the broiler for a minute or two to crisp the topping.