Preheat oven to 350°F (175°C).

Cook spaghetti according to package directions. Drain and set aside.

While spaghetti cooks, sauté chopped onion and sliced mushrooms in butter until softened.

In a separate pot, melt butter, then whisk in flour until smooth.

Gradually whisk in milk until smooth. Bring to a simmer.

Stir in grated nutmeg and sherry. Simmer until slightly thickened.

If desired, add blanched broccoli to the sauce.

Combine cooked spaghetti, sautéed onion and mushrooms, and the sherry sauce in a large bowl.

Pour mixture into a buttered 9x13 inch baking dish.

Top with Parmesan cheese.

Bake for 45-60 minutes, or until bubbly and golden brown.