Preheat the oven to 350 degrees Fahrenheit.
Cook all the spaghetti in boiling salty water, just until al dente. It will continue cooking in the oven. Drain the pasta and set it aside.
In a large pot, melt the butter and sauté the minced garlic cloves in it for 4-5 minutes on medium heat.
Cook for a few minutes until the mushrooms release some water before adding the salt, pepper, and nutmeg.
Add the vinegar and continue simmering the mixture to reduce the liquid even more.
Combine the flour, chicken broth, and milk. Cook it while stirring until it thickens.
Combine the spaghetti with the chicken thigh pieces, the mushroom mixture, bacon pieces, and mix them well.
Transfer everything to a large baking dish, and sprinkle all the grated cheese on top.
Bake the Chicken Tetrazzini in the 350 degrees preheated oven for 20-25 minutes until it starts to bubble and the cheese turns slightly golden brown and melted.
When plated, you can garnish the meal with some fresh parsley.
Serve it hot!