Combine ground turkey, panko breadcrumbs, grated Parmesan cheese, beaten egg, finely chopped onion, minced garlic, chopped fresh sage, dried thyme, salt, and black pepper in a large mixing bowl. Mix gently.

Shape the mixture into 1.5-inch meatballs and place them on a parchment-lined baking sheet.

Heat olive oil in a large skillet over medium heat. Add meatballs in batches and cook until golden brown, about 5-7 minutes.

In the same skillet, add more olive oil if needed and sauté chopped onion for 3-4 minutes until translucent. Add minced garlic and sauté for 1-2 minutes.

Stir in pumpkin puree, chicken broth, heavy cream, additional Parmesan cheese, chopped sage, thyme, and nutmeg. Whisk until smooth.

Bring the sauce to a gentle simmer and let it cook for 10-15 minutes, stirring occasionally.

Taste the sauce and adjust seasoning with salt and pepper as needed.

Add the browned meatballs to the simmering sauce, cover, and cook for an additional 15-20 minutes.

Serve the meatballs hot over pasta, polenta, or mashed potatoes. Garnish as desired.