Heat 1 tablespoon of olive oil in a large skillet over medium-high heat. Once hot, add sliced onions, stirring to coat thoroughly. Cook for about 10 minutes, stirring constantly, until they start to soften.

Reduce heat to medium, sprinkle in a pinch of salt, and continue cooking onions for an additional 20 minutes until they turn a deep golden color.

Stir in 1 tablespoon of balsamic vinegar to deglaze the pan, cooking for another minute before transferring caramelized onions to a plate.

In the same skillet, heat another tablespoon of olive oil over medium heat, add sliced mushrooms with a pinch of salt, and cook for approximately 5 minutes until tender.

Stir in minced garlic and cook for 1-2 minutes, then add fresh spinach, tossing until spinach wilts.

If making the creamy sauce, add ⅓ cup of heavy cream and ¼ cup of milk to the skillet, bringing it to a boil. Reduce to simmer to thicken.

Mix in ½ cup of shredded Parmesan cheese, stirring until it melts into the sauce.

Return caramelized onions to the skillet, gently folding into the mixture. Allow flavors to meld for another minute before serving.