Preheat the oven to 425 degrees. Drizzle the mushrooms with olive oil, then sprinkle them with salt and pepper. Roast them in the oven until deep golden brown, 20 to 25 minutes. Set aside.
Next, fry the bacon until chewy but not yet crisp. Chop the bacon into bite-size bits and set aside.
In a large skillet over medium-low heat, saute the onions in 1 tablespoon butter, stirring occasionally, until golden brown, about 15 minutes. Set aside.
Grate the Parmesan, Gruyere, and fontina cheeses. Unwrap the goat cheese.
Cook the macaroni until just undercooked. Drain and set aside.
To make the sauce, melt the remaining 4 tablespoons butter in a large pot over medium heat. Sprinkle in the flour, whisking to combine. Let the roux cook for a minute or so, whisking constantly. Pour in the milk, whisking constantly, then cook the white sauce for 3 to 5 minutes, or until thick and bubbly.
Next, add the half and half, 1 teaspoon of salt, plenty of black pepper, and stir to combine.
Spoon about 1/4 cup of the hot white sauce into a small bowl containing the beaten egg, stirring the egg with a fork as you drizzle it in. Once combined, pour the mixture into the pot with the white sauce, stirring constantly. Add all the cheese in and stir them around to melt.
Add the cooked macaroni and stir to coat. Splash in a little milk or hot water as needed for thinning.
Preheat the oven to 350 degrees. Butter a 9 x 13 inch baking pan generously.
Lay half the cooked onions in the bottom of the pan, followed by half the macaroni, half the roasted mushrooms, half the Gorgonzola, and half the bacon. Repeat the layers, ending with the bacon. Bake for 20 to 25 minutes, or until bubbly and hot. Delicious!
