• Preheat oven to 425 Fahrenheit, with rack positioned in the center of the oven. Line a sheet pan with parchment and spread 1 tablespoon of oil on the parchment. Set aside.

Prepare the Cauliflower:

• Arrange the trimmed and cut cauliflower on the sheet pan with the diced onions distributed evenly and in one layer. Once the oven is preheated, put the sheet pan in the oven and set the timer for 15 minutes.
• Check on the cauliflower after 15 minutes. Give the pan a shake and gently, using a spatula, redistribute the cauliflower again in one layer. Set the timer for another 5 minutes then check again. The cauliflower should be turning a nice golden color with bits of dark brown. Roast for an additional 5 minutes if needed.

  Redistribute the cauliflower again, but this time to the center of the pan. It's ok if it's piled up. Add 1/4 cup Parmesan and 1/2 cup Cheddar and bake for an additional 8-12 minutes or until the cheese is thoroughly melted. Remove the pan from the oven and set aside.

Cook the Pasta:

• Meanwhile, put a pot of water with a heavy pinch of salt on to boil for the pasta shells. Do this in a 3 1/2 - 4 quart (3.3 - 3.7 Liter) Dutch oven if you can (for fewer pots to clean). Add the pasta once the water is boiling and boil until tender (or as indicated package directions). Drain pasta and set aside. It's okay if it sticks together. If using a Dutch oven, there's no need to clean it, unless pasta has stuck to the bottom. You'll make the sauce in it too.
• Reduce the temperature of the oven to 350 Fahrenheit (176 Celsius).

  Spray the bottom and sides of a baker (8 1/2 inch x 11 inch or 8 inch x 8inch / 21.6 x 28 centimeters or 20 x 20 centimeters) and/or ramekins with pan spray. Set aside. Otherwise, continue making the recipe in the Dutch oven to go from stovetop to oven.

Make the Sauce:

• Warm the milk in the microwave in a heatproof, spouted bowl or measuring cup until steaming. Set aside. To the Dutch oven, or medium sauce pot add the remaining 1 tablespoon oil and 1 tablespoon of butter. Melt and allow the butter to bubble. Turn the burner to low. Toss in the microplaned garlic and push it around the pan using a whisk for no more than 10 seconds. Sprinkle in the flour and whisk constantly until a loose paste is formed. Now you're ready to add the milk, a little at a time while whisking the entire time.

  Continue to add a little more milk at a time while the sauce begins to thicken slightly and become smooth, with little to no lumps. Once all the milk is in the pot and your sauce is smooth, add the salt, pepper, and the remaining cheese. A few glugs of tabasco to taste is especially nice here. Stir until completely smooth and thickened. Remove the pot from the heat.

Finish and Bake the Mac and Cheese:

• To the sauce, stir in the cooked pasta, roasted cauliflower, and the little bits of crunchies on the pan. If using a Dutch oven, wipe the top clean around the rim and continue with the breadcrumbs. If using a baker, transfer the mixture to the prepared baker.

  Cover the mixture with breadcrumbs. Sprinkle paprika, a pinch of salt and a few grinds of pepper over the top, to taste. A little paprika goes a long way.
• Bake uncovered for about 20 minutes or until the breadcrumbs are toasty and the mac and cheese is heated through. Check after 10 minutes to make sure the bread crumbs aren't getting too dark. If so, simply lay a piece of foil over the top. If baking in a Dutch oven, you'll need to give the casserole about 5-10 more minutes.
• Garnish with fresh thyme. Serve.

To Store and Reheat:

• Store in a lidded container in the refrigerator for up to three days. Freeze for up to two weeks. Thaw overnight in the fridge. Gently rewarm stovetop with a splash or two of milk to loosen and soften things up.