Spinach, Mushroom, and Caramelized Onion Pasta Recipe

Caramelize onions by melting butter in a large, nonstick skillet over medium-high heat. Then, toss in onions and cook for about 20-30 minutes, or until the onions are caramelized. Make sure to stir continuously to avoid onions from sticking to the bottom of the pan, adding in broth tbsp by tbsp as needed to avoid burning and sticking. Right before the onions are finished, add in balsamic vinegar, and stir the onions, scraping up all the bits at the bottom of the pan. Remove onions and set aside.
Prepare pasta according to package directions.
While pasta is cooking, use the same pan that was used to caramelize the onions, and heat oil in it over medium-high heat. Add in garlic and sauté until fragrant, about 1-2 minutes. Stir in sliced mushrooms and sprinkle with a bit of salt. Turn heat to medium, and cook, stirring frequently, until the mushrooms have softened, about 15 minutes.
Return onions to the pan and then stir in milk and parmesan cheese. Mix well and season with salt and pepper as desired. Cook over medium heat, stirring frequently until the parmesan is fully melted and the sauce is well combined.
Drain pasta, and place hot noodles in a large bowl. Stir in the fresh spinach leaves, and toss well with the hot pasta until the spinach starts to wilt.Pour the mushroom and onion sauce mixture over the pasta noodles and mix well. Add additional salt and pepper as needed.