Cheesy Mushroom au Gratin

6 tablespoons (¾ stick) butter, divided
½ cup all-purpose flour
6 cups (1½ quarts) milk, heated
1 cup (4 ounces) shredded Colby cheese
1 cup (4 ounces) shredded Muenster cheese
1½ teaspoons salt
1½ teaspoons red pepper sauce
½ teaspoon ground nutmeg
3 pounds fresh white mushrooms, sliced
2¼ pounds fresh shiitake mushrooms, sliced
1 cup green onions, sliced
1 tablespoon dried fines herbes
24 slices assorted breads (such as rustic white, whole wheat, and sourdough), toasted
¾ cup (about 3 ounces) grated Asiago cheese
Asparagus spears, for serving

A rich and delicious casserole for mushroom lovers, made with fresh mushrooms and cheese.

Preheat the oven to 400°F.
In a large skillet, heat 4 tablespoons of the butter; whisk in the flour to create a roux. Cook on low heat for 3 to 4 minutes. Slowly whisk in the hot milk, stirring well between additions. Bring the sauce to a boil; then reduce to a simmer. Cook 5 to 6 minutes, stirring occasionally. Add the Colby and Muenster cheeses and stir well until melted. Season with the salt, red pepper sauce, and nutmeg; set aside.
In a large skillet, heat the remaining 2 tablespoons of butter; add the mushrooms, green onions, and fines herbes. Cook on high heat until the mushrooms are golden brown, about 10 to 12 minutes. Stir into the cheese sauce.
Place 2 slices of toasted bread in each of 12 shallow, lightly buttered ramekins or oven-to-table dishes; top each serving with ½ cup of mushroom sauce, and sprinkle with 1 tablespoon of the Asiago. Bake for 8 to 10 minutes, or until brown and bubbly.
To serve, top with the asparagus spears and the remaining shredded Asiago cheese.