Start by melting 3 tablespoons of butter in a large skillet over medium heat. Add ⅔ cup of finely chopped yellow onion, 3 minced garlic cloves, and 3 cups of sliced mushrooms. Sauté this mixture for about 5 minutes, or until the onions are translucent and the mushrooms are tender and lightly browned.

Next, add 16 ounces of baby spinach to the skillet along with ¼ teaspoon of salt and ½ teaspoon of black pepper. Stir the mixture occasionally for about 2 minutes, or until the spinach is wilted.

Once the spinach has wilted, carefully transfer the sautéed mixture into your slow cooker. Scrape all the flavorful bits from the skillet.

Pour in 5 cups of vegetable or chicken broth and 1.5 cups of half and half into the slow cooker. Sprinkle in ½ teaspoon of Italian seasoning, ¼ teaspoon of thyme, and 2 teaspoons of garlic powder, stirring gently to combine.

Cover the slow cooker and set it to cook on high for 3 hours. For firmer tortellini, wait to add 16 ounces of frozen cheese tortellini until the last 30 minutes.

After 3 hours, check the soup for seasoning, adjusting with more salt and pepper if needed. Whisk ⅓ cup of flour with 1 cup of milk and stir it in for extra creaminess. Garnish with ½ cup of grated Parmesan cheese before serving.