Step 1

Heat the olive oil in large skillet over medium heat. Add the mushrooms and scallions and sauté until the mushrooms are soft and golden. Set aside.

Step 2

Melt the butter in a heavy 4-quart saucepan over medium heat. Add the flour and cook, stirring, for 1 minute. Add the milk, salt, and pepper and stir vigorously with a wire whisk until the mixture barely comes to a boil. Reduce the heat and simmer for 5 minutes, whisking frequently while the sauce thickens. Add the Fontina, bacon, and sauteed vegetables, and stir until the cheese melts. Keep warm.

Step 3

Cook the spaghetti according to the package directions. Drain well.

Step 4

Add the spaghetti and parsley to the sauce, mix well, and transfer to a serving dish. Sprinkle with Parmesan and serve immediately.