3 tbsp. olive oil, divided, plus more for baking dish
1 lb. cavatappi or other short pasta
1 medium-sized head cauliflower (about 2 pounds), cored and thinly sliced
4 cloves garlic, sliced
1 large yellow onion, thinly sliced
Kosher salt and freshly ground black pepper
4 oz. extra-sharp white Cheddar cheese, grated (about 1 cup)
2 oz. Parmesan cheese, grated (about 1/2 cup)
1/4 tsp. mustard powder
Pinch cayenne pepper
1 1/2 c. panko breadcrumbs
1/2 c. fresh flat-leaf parsley, chopped
1.Preheat Oven To 425°F. Lightly Oil A 9-By-13-Inch Baking Dish. Cook Pasta According To Package Directions. Drain.
2.Heat 2 Tablespoons Oil In A Large Pot Over Medium Heat. Add Cauliflower, Garlic, And Onion. Season With Salt. Cook, Covered, Stirring Occasionally, Until Tender, 15 To 20 Minutes. Add 4 Cups Water And Simmer Until Vegetables Are Very Soft, 10 To 12 Minutes. Drain, Reserving 2 Cups Cooking Liquid; Let Cool Slightly.
3.Combine Vegetables, Cheddar, Parmesan, Mustard Powder, And Cayenne In A Blender (Depending On The Size Of Your Blender, You May Need To Do This In Two Batches). Purée, Adding Just Enough Reserved Cooking Liquid To Get Mixture Moving, Until Smooth, 1 To 2 Minutes.
4.Add Sauce To Pasta And Toss To Combine. Transfer To Prepared Baking Dish. Toss Together Panko, Parsley, And Remaining Tablespoon Oil In A Bowl. Season With Salt And Pepper. Sprinkle Over Pasta. Bake Until Golden Brown, 14 To 16 Minutes.