01 - Preheat oven to 400°F (200°C). Toss butternut squash cubes with olive oil, salt, and pepper. Spread on a baking sheet and roast for 25-30 minutes until tender and caramelized. Allow to cool slightly, then purée until smooth using a blender or immersion blender.
02 - Cook the pasta in salted boiling water until just al dente. Drain and reserve.
03 - Melt 2 tbsp butter in a large saucepan over medium heat. Whisk in flour and cook for 1-2 minutes until foamy but not browned. Gradually whisk in the warm milk, stirring constantly until smooth and thickened, approximately 3-4 minutes.
04 - Reduce heat to low and add the grated cheddar and Gruyère cheeses, stirring until fully melted and smooth. Stir in the butternut squash purée and nutmeg. Season with salt and pepper to your preference.
05 - Gently combine the cooked pasta with the cheese sauce, ensuring each piece is evenly coated. Transfer to a serving dish or individual bowls.
06 - Melt 0.25 cup butter in a small saucepan over medium heat until it foams and turns golden brown, swirling occasionally. Add sage leaves and fry for 1-2 minutes until crisp. Remove sage leaves and set aside on paper towel-lined surface.
07 - Add milk, heavy cream, and a pinch of salt to the brown butter. Warm gently, taking care not to boil. Use a handheld milk frother or immersion blender to blend until a light foam forms.
08 - Top each portion of mac and cheese with a generous spoonful of brown butter sage foam and garnish with the crispy sage leaves.