01 - Bring a large pot of salted water to a rolling boil. Add the pasta and cook according to package directions until al dente. During the final 4 minutes of cooking, incorporate the broccoli florets into the boiling pasta water. Drain thoroughly, reserving a small amount of pasta water if desired.
02 - In a large skillet, melt the unsalted butter over medium heat. Add the finely chopped onion and sauté for 2 to 3 minutes until softened and translucent. Stir in the minced garlic and cook for an additional minute until fragrant.
03 - Sprinkle the all-purpose flour into the skillet with the sautéed aromatics. Whisk continuously for 1 minute to form a smooth roux, ensuring the raw flour taste is cooked out.
04 - Gradually pour in the whole milk, whisking constantly to prevent lumps from forming. Incorporate the heavy cream and bring the mixture to a gentle simmer. Continue to cook, stirring occasionally, until the sauce begins to slightly thicken, approximately 3 to 4 minutes.
05 - Stir in the grated Parmesan cheese, 1/2 teaspoon salt, 1/4 teaspoon ground black pepper, and the optional pinch of nutmeg. Continue stirring until the cheese has melted completely and the sauce is smooth and homogenous.
06 - Add the drained pasta and broccoli to the creamy sauce in the skillet. Toss thoroughly to ensure all components are evenly coated. If the sauce appears too thick, incorporate a splash of the reserved pasta water to achieve the desired consistency. Serve immediately, garnished with additional grated Parmesan, freshly cracked black pepper, and chopped parsley, if desired.