Tortellini Soup with Balsamic Caramelized Onions & Mushrooms

The onions:
Heat olive oil in a large saucepan set over medium heat. Add the onions and cook until softened, stirring occasionally, about 10 minutes. Cover and cook until the onions are golden brown, 8 to 10 minutes, stirring occasionally.

Add 1/4 cup water and thyme, cover the pan again and cook until the onions are deep golden brown, about 15 minutes. Stir in balsamic vinegar, salt, and the additional 1/4 cup water. Cover the pan and cook until the liquid is absorbed. Then remove from heat and set aside.

The mushrooms:
Heat olive oil in a large skillet set over medium heat. Add mushrooms. Cook until the mushrooms are tender, stirring frequently, about 5 minutes. Add balsamic vinegar, salt and pepper, and stir to combine. Then add them to the onions in a large soup pot.

The soup:
Add the chicken broth to the onions and mushrooms. Stir well. Bring the mixture to a boil over medium-high heat, then lower to a simmer. Add the fresh or frozen cheese tortellini. Simmer for 5 minutes for fresh or 8 minutes for frozen tortellini. The tortellini should be just tender. Season with salt and pepper to taste. Stir in spinach at the last minute. Serve, sprinkled with grated Parmesan cheese, if desired.