Place an oven rack in the center of the oven. Heat the oven to 375° F.
1⅓ cup uncooked macaroni
Cook macaroni al dente, being sure to reserve ⅓ cup of the pasta water before draining and rinsing with cold water. Set cooked pasta aside.
Heat a large oven safe pan over medium-low heat. Add the oil or butter and swirl to coat the bottom of the pan.
2 TBS olive oil or butter
Add the onions to the pan. Sauté until translucent, about 5 minutes. Add the garlic and cook for 30 seconds, stirring often to prevent burning.
½ yellow onion, roughly chopped, 3 cloves garlic, roughly chopped
Add the cauliflower, nutmeg, and milk to the pan. Carefully stir to combine. Bring to a simmer, cover, and let simmer for 10 minutes or until the cauliflower is easily pierced with a knife.
1 small head cauliflower, cut into medium florets, 2 cups milk, ¼ tsp nutmeg
Once the cauliflower is soft, turn off the heat and use an immersion blender to blend the cauliflower until smooth.
While the blended cauliflower is still hot, add half of both the cheddar cheese and parmesan cheese. Stir until the cheeses have fully melted and incorporated into the sauce.
2 cups grated cheddar cheese, divided, ½ cup grated parmesan cheese, divided
Stir in the reserved pasta water. Fold in the cooked macaroni.
Sprinkle the rest of the cheese in an even layer over the top.
Place the dish into the oven. Bake, uncovered, for 20 minutes or until browned and bubbly on top.
Remove from the oven, let cool for 10 minutes, then enjoy.