Bring a large pot of salted water to a boil. Cook pasta according to package directions, reserving ½ cup of pasta water before draining. Set aside.
While pasta cooks, prepare all ingredients: slice mushrooms, dice onion, and chop bacon.
In a large Dutch oven or deep frying pan, cook chopped bacon over medium-high heat for about 6 minutes or until crisp. Remove bacon to a paper towel–lined plate.
Drain most of the bacon grease, leaving a thin layer in the pan. Add 1 tablespoon butter and chicken cutlets. Season with salt and pepper and cook for about 3 minutes per side, or until cooked through. Set aside.
Reduce heat to medium. Melt remaining 2 tablespoons butter, then add mushrooms and onions. Cook for about 3 minutes, stirring often. Add garlic and cook 1 minute more.
Sprinkle flour over the vegetable mixture. Gradually whisk in ½ cup chicken broth until smooth, then add remaining broth, heavy cream, ranch seasoning, and Parmesan. Season to taste.
Bring the sauce to a gentle simmer, stirring until thickened. Add drained pasta, chopped chicken, and crumbled bacon. Toss to coat, thinning with reserved pasta water as needed.
Adjust seasoning if necessary and serve warm.
Notes
Cook Bacon First. The rendered bacon fat is mixed with a bit of butter and used to cook the chicken cutlets, adding an extra layer of flavor.
Whisk constantly after adding the flour and liquids. As it simmers it will thicken into a smooth creamy sauce.
Reserve some pasta water before draining. Pasta water is great for thinning out and binding pasta sauce.