Preheat oven to 375°F (190°C). Grease a 9x13 inch baking dish. Trim the woody ends off the asparagus and cut the spears into 1-inch pieces. Slice the cremini mushrooms. Finely dice the yellow onion and mince the garlic.

In a large skillet over medium-high heat, add the olive oil. Once hot, add the sliced mushrooms and sauté for 5-7 minutes until they release their liquid and start to brown. Add the diced onion and asparagus pieces and continue to cook for another 3-4 minutes until the asparagus is bright green and slightly tender-crisp. Stir in the minced garlic and cook for 1 minute more until fragrant. Remove vegetables from the skillet and set aside.

In the same skillet, melt 4 tablespoons of unsalted butter over medium heat. Whisk in the all-purpose flour and cook for 1-2 minutes, stirring constantly, to create a roux.

Gradually whisk in the chicken broth, then slowly add the warmed milk, whisking continuously to prevent lumps. Bring the sauce to a gentle simmer, stirring frequently, until it thickens, about 3-5 minutes.

Reduce heat to low. Add the softened cream cheese cubes, Dijon mustard, ground nutmeg, salt, and black pepper. Whisk until the cream cheese is fully melted and the sauce is smooth. Stir in the Gruyère cheese until melted and incorporated.

Return the sautéed asparagus, mushrooms, and onions to the skillet with the cream sauce. Stir gently to ensure all vegetables are coated. Pour the entire mixture into the prepared 9x13 inch baking dish, spreading it evenly.

In a small bowl, combine the Panko breadcrumbs, grated Parmesan cheese, and 2 tablespoons of melted butter. Mix well until the breadcrumbs are evenly coated.

Sprinkle the Panko topping evenly over the casserole mixture. Bake for 25-30 minutes, or until the casserole is bubbly around the edges and the Panko topping is golden brown and crispy.

Remove from oven and let stand for 5-10 minutes before serving.