In a skillet, heat 1 tablespoon of olive oil over medium heat. Add the sliced onions and a pinch of salt. Cook, stirring occasionally, for about 15-20 minutes until the onions are golden brown and caramelized.

Remove the caramelized onions from the skillet and set aside.

In a large pot, bring salted water to a boil. Add the farfalle pasta and cook according to package instructions until al dente, about 10-12 minutes.

Reserve 1 cup of pasta water, then drain the pasta in a colander and set aside.

In the same skillet used for the onions, add 1 tablespoon of butter and heat over medium heat. Add the sliced mushrooms and cook for about 5-7 minutes until they are tender.

Add the minced garlic and chopped spinach to the skillet. Sauté for an additional 2-3 minutes until the spinach is wilted.

Add the cooked farfalle pasta to the skillet with the sautéed vegetables. Toss to combine, adding reserved pasta water as needed to create a light sauce.

Stir in the caramelized onions, and season with salt and pepper to taste. Serve hot, topped with grated Parmesan cheese.