Start by turning your Instant Pot to the “Sauté” setting and melting the butter. Once melted, add the minced garlic and sauté for about 1 minute, stirring frequently, until fragrant but not browned.
Place the boneless, skinless chicken breasts right into the pot. Pour in the 4 cups of low sodium chicken stock, then season with salt, black pepper, and nutmeg if using. Give everything a gentle stir to combine.
Secure the lid on your Instant Pot and set the valve to sealing. Cook on high pressure for 10 minutes. When the cooking cycle is complete, allow a natural pressure release for 10 minutes before carefully releasing any remaining pressure manually.
Open the lid and remove the chicken breasts. Using two forks, shred the chicken into bite-sized pieces. Return the shredded chicken to the pot.
Add the fresh tortellini directly to the hot broth. Close the lid and set the Instant Pot to the “Sauté” function again. Cook the tortellini for 3-4 minutes, stirring occasionally to prevent sticking, until tender and cooked through.
Reduce the heat to low. Stir in the heavy whipping cream and shredded parmesan cheese until fully incorporated and the soup is creamy. Then add the cornstarch slurry (cornstarch mixed with water) to the pot, stirring constantly until the soup thickens slightly, about 2 minutes.
Finally, stir in the fresh spinach. Let it wilt in the warm soup for about 1-2 minutes. Taste and adjust seasoning with additional salt or pepper if needed.
Ladle the soup into bowls and garnish with extra parmesan cheese if desired. Serve hot with crusty bread or a side salad for a complete meal.