Preheat oven to 400F and line a large sheet pan with parchment paper. Toss cauliflower florets with olive oil, salt, and pepper to taste and arrange in a single layer on sheet pan. Roast cauliflower at 400F 15-20 minutes until nearly tender and a light golden brown. Keep oven on and set cauliflower aside.
In a medium saucepan over medium heat, melt butter. Whisk in garlic and flour and let cook 2-3 minutes until golden brown. Slowly drizzle in milk and cream, whisking constantly until smooth and incorporated.
Cook sauce over medium heat, stirring occasionally until sauce is bubbly and thickened. Remove from heat and add cheeses, stirring until melted.
Fold roasted cauliflower into sauce and season mac and cheese with salt and pepper to taste. Scrape mixture into a greased 9-inch baking dish and top with panko breadcrumbs.
Bake mac and cheese at 400F 20-25 minutes until bubbly. Optionally, broil mac and cheese 2-3 minutes on high to brown breadcrumbs. Serve mac and cheese warm and enjoy!