Adjust an oven rack to the middle position and preheat the oven to 375 degrees F.
Grease a rimmed sheet pan with the room-temperature butter and set aside. Combine the breadcrumbs and Parmigiano-Reggiano cheese in a medium-size bowl. Stir in the melted butter and set aside.
Whisk the whole milk, evaporated milk, cream, onion, garlic powder, salt, and the Cheddar and fontina cheeses in a large bowl. Add the uncooked macaroni and stir until well coated. Pour the mixture into the prepared sheet pan, spreading it in an even later. Cover the sheet pan with aluminum foil (you may need 2 sheets), crimping it tightly around the edges to seal (make sure to get a good seal on the foil, otherwise the pasta won't cook). Bake for 25 minutes.
Remove the foil and stir the macaroni. Sprinkle the bread crumb mixture over the top and continue to bake for 25 minutes more.
Turn the broiler to high and broil the macaroni and cheese until it is golden brown on top and bubbling, about 10 minutes (watch the macaroni and cheese closely, as broiler intensities vary). Remove from the oven and let the pasta stand, covered, 5 minutes before serving.
"Sheet Pan Suppers Meatless: 100 Surprising Vegetarian Meals Straight from the Oven" by Raquel Pelzel © Workman Publishing Company 2017. Provided courtesy of Raquel Pelzel. All rights reserved.