Preheat the oven to 375°F (190°C).

Bring a large pot of salted water to a boil. Cook pasta until just al dente, about 9-11 minutes. Drain and toss with a little oil to prevent sticking.

In a large skillet, melt butter over medium heat. Add chopped onion and cook until translucent, about 4 minutes. Stir in minced garlic and cook for 30 seconds until fragrant.

Sprinkle flour over the butter and onion mixture. Stir constantly for 1-2 minutes to cook out the raw flour taste.

Slowly whisk in warmed milk, ensuring no lumps form. Bring to a gentle simmer and cook until sauce thickens, about 5-7 minutes. Add Dijon mustard, nutmeg, salt, and pepper to taste.

Remove sauce from heat and stir in Parmesan cheese until melted and smooth.

Blanch asparagus in boiling water for 2 minutes, then shock in ice water. Drain well.

Combine cooked pasta, ham, asparagus, shredded mozzarella, and creamy sauce in a large bowl or directly in the baking dish. Stir gently to coat evenly.

Transfer mixture to a greased 9×13 inch casserole dish. Evenly sprinkle grated Gruyere cheese on top.

Bake uncovered for 20-25 minutes until cheese topping is bubbly and golden brown. For extra crispiness, broil for the last 2 minutes, watching carefully to avoid burning.

Let the pasta bake rest for 5 minutes before serving.