My mac & cheese moment

Cremini mushrooms, cleaned, quartered	1 pound
Oil, olive	Drizzle
Salt, kosher	½ teaspoon +
Pepper, freshly ground
Sausage, sweet Italian, chopped	2
Onions, halved, thinly sliced	2 large
Butter, unsalted	1 tablespoon + 4 tablespoons
Parmesan, grated	½ cup
Edam, grated	½ cup
Roily Run cheese, grated	½ cup
Fromager d’affinois, room temperature	½ cup
Macaroni	1 ½ pounds
Flour, all-purpose	¼ cup
Milk, whole	2 cups
Half-and-half	½ cup
Eggs, lightly beaten	2 large
Gorgonzola, crumbled	½ cup
Bread crumbs, fine	½ cup

1. Preheat the oven to 425°F. 
2. Drizzle the oil over the mushrooms, and season with salt and pepper. 
3. Roast until a deep golden brown, about 20 to 25 minutes; set aside. 
4. Preheat the oven to 350°F; grease a 9-by-13-inch pan generously with butter. 
5. In a medium skillet, brown the sausage over moderate heat; set aside. 
6. In a large skillet over medium-low heat, melt the 1-tablespoon butter, and sauté the onions, stirring occasionally, until golden, about 15 minutes; set aside. 
7. Cook the macaroni until just undercook; drain and set aside. 
8. In a large pot over medium heat, melt 4-tablespoons butter. 
9. Sprinkle in the flour, whisking to combine; cook for 3 to 5 minutes, whisking constantly. 
10. Gradually pour in the milk, whisking constantly, and cooking until thick and bubbly, about 3 to 5 minutes. 
11. Add the half-and-half, ½-teaspoon salt, pepper, and stir to combine. 
12. In a small bowl with the eggs, gradually add about ¼-cup sauce, stirring constantly, and then the mixture back to the sauce, stirring even more. 
13. Add all the cheeses except for the Gorgonzola, and stir until melted. 
14. Add the macaroni, stir to coat; add a splash of milk, as needed, to thin the sauce. 
15. Layer half the onions on the bottom of the greased baking dish, followed by half the macaroni, half the mushrooms, half the Gorgonzola, and half the sausage. 
16. Repeat the layers, ending with the last of the sausage. 
17. Sprinkle bread crumbs on top.
18. Bake for 20 to 25 minutes, or until bubbly and hot.