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.

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.

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

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