In a large pot over medium heat, bring about 4 quarts salted water to a boil. Add spaghetti and cook 1 minute shy of package directions. Remove from heat and drain well. Keep warm.
In a wide pan over medium heat, add bacon. Cook, stirring occasionally, until it browns and crisps. With a slotted spoon, remove from heat and drain on paper towels.
Discard the rendered fat except for about 1 tablespoon. Add onions and garlic and cook until softened.
Add mushrooms and cook until softened.
With a slotted spoon, remove the mushroom mixture from the pan and keep warm.
Add butter to pan and heat until melted. Add flour and cook, stirring regularly, for about 1 to 2 minutes or until smooth and lightly browned.
Slowly add chicken broth, whisking regularly to prevent lumps.
Add cream. Lower heat and bring to a simmer. Continue to cook, whisking occasionally, until slightly thickened.
Add Parmesan cheese and nutmeg. Season with salt and pepper to taste.
Add pasta, bacon, and mushroom mixture. Toss to combine until pasta is coated with sauce and heated through.
To serve, garnish with fresh parsley and additional Parmesan cheese. Serve hot.