In a large mixing bowl, combine ground turkey, panko breadcrumbs, Parmesan cheese, beaten egg, yellow onion, minced garlic, fresh sage, thyme, salt, and pepper. Mix gently to combine, avoiding overmixing.

Form the mixture into 24 small meatballs, about 1 inch in diameter, and place on a lined baking sheet.

Heat olive oil in a skillet over medium heat and brown the meatballs for 2-3 minutes on each side, then remove and set aside.

In the same skillet, add more olive oil if needed, then add chopped yellow onion and sauté until translucent, about 5 minutes. Add minced garlic and cook for 1 minute.

Stir in pumpkin puree and chicken broth, bring to a simmer and let cook for 5 minutes to meld flavors.

Reduce heat to low, add heavy cream, Parmesan cheese, fresh sage, thyme, and nutmeg, and season with salt and pepper. Cook for 2-3 minutes until creamy.

Let the sauce thicken for about 5-10 minutes, stirring occasionally.

Add the browned meatballs back into the sauce, ensuring they are submerged, and allow them to warm through.

Cover and let simmer on low for 15-20 minutes until meatballs reach an internal temperature of 165°F (74°C).

Serve the meatballs over pasta or polenta, garnished with fresh sage and Parmesan cheese.