01 - Preheat oven to 400°F. Line a baking sheet with parchment paper.
02 - Brush squash halves with olive oil and season with kosher salt and black pepper. Place cut-side down on the prepared sheet.
03 - Slice the top off the garlic head to expose the cloves. Drizzle with olive oil and sprinkle with a pinch of salt. Wrap tightly in aluminum foil.
04 - Roast squash and garlic for 40 to 45 minutes until squash is fork-tender and garlic is golden and soft. Remove from oven and set aside.
05 - While roasting, melt butter in a saucepan over medium heat. Whisk in flour and cook for 1 minute.
06 - Gradually whisk in whole milk and heavy cream. Stir constantly until slightly thickened, about 3 to 4 minutes.
07 - Squeeze roasted garlic cloves from their skins, mash, and whisk into the sauce.
08 - Add Parmesan, mozzarella, fontina, and Gorgonzola cheeses. Stir until melted and smooth. Season sauce with black pepper, nutmeg, and salt to taste. Remove from heat.
09 - Carefully flip squash halves right-side up. If necessary, scoop out some flesh to enlarge the cavity and reserve any excess for another use.
10 - Spoon Alfredo cheese sauce into each squash half.
11 - Return stuffed squash to oven and bake for 10 to 12 minutes until sauce is bubbling and golden.
12 - Sprinkle with chopped parsley and extra Parmesan before serving.