-Melt 3 TBS butter in large pot over medium heat.
-Chop and add 1 onion and sauté until translucent, about 5 minutes.
-Stir in 3 minced garlic cloves, then ¼ cup flour; stir constantly 1 minute.
-Gradually whisk in 3 cups whole milk.
-Cook, whisking occasionally, until mixture begins to boil, about 5-7 minutes.
-Add 2 cups coarsely grated sharp white cheddar. Stir until cheeses melt, about 2 minutes.
-Stir in ¼ tsp cayenne and 1/8 tsp ground nutmeg.
-Season sauce to taste with salt and pepper.
-Preheat oven to 350°F. Butter a 13x9x2-inch baking dish.
-Boil a large pot of water.
-Cut 1 head of cauliflower and/or some broccoli into small florets.
-Add the cauliflower/broccoli and cook for 3-5 minutes until just a little tender.
-Scoop out the cauliflower/broccoli and return water to a boil.
-Add 12 ounces macaroni and cook according to package directions until tender but still firm to bite.
-Then drain the macaroni.
-Stir macaroni and cauliflower/broccoli into warm cheese sauce.
-Place half of macaroni in buttered baking dish; smooth top.
-Top with 1 more cup cheddar cheese.
-Top with remaining macaroni mixture; spread evenly.
-Crumble/tear up 1-2 pieces of bread so that you have 1 cup fresh breadcrumbs.
-Melt 3 tablespoons butter.
-Place breadcrumbs in medium bowl. Drizzle butter over; toss.
-Add ½ cup finely grated Parmesan and sprinkle with salt and pepper; toss.
-Sprinkle breadcrumb mixture over mac and cheese.
-Bake mac and cheese until breadcrumbs are golden and edges are bubbling, about 40 minutes.
-Let stand 10 minutes before serving.