Preheat oven to 450ºF. Place the cauliflower and carrots on a baking tray and toss with the oil and salt and pepper to taste. Roast, turning the vegetables once during cooking, until golden brown (about 15 minutes). Reserve.
While the veggies are cooking, bring a pot of salted water to a boil. Cook the pasta al dente according to package directions. Drain pasta and run under cold water to stop the cooking.
In a small bowl, combine the breadcrumbs, thyme, and 1 tsp. of the garlic. Heat a large saucepan over medium low heat. Melt the butter. Remove 1 Tbsp. and add to the breadcrumbs. Stir to combine and reserve.
Add the remaining garlic to the saucepan and cook until the garlic is aromatic (30 seconds). Whisking constantly, stir in the flour, taking care the flour doesn’t brown. Continue to cook for 2 minutes.
Whisk in the milk. It will thicken as it gets hot. Whisk constantly; any lumps will smooth out. Cook for 5 – 7 minutes, mixture will bubble and thicken. When it no longer changes in consistency, it’s done.
Stir in nutmeg, cayenne, and cheese. Season to taste with salt and pepper. Stir in pasta, carrots, and cauliflower. Spread into a 9” x 13” pan. Sprinkle with the breadcrumb mixture. (Mac ‘n’ Cheese can be made ahead to this point and refrigerated or frozen.)