In a bowl, soak the breadcrumbs with milk for a couple of minutes. Stir in onion, garlic, sage, parsley, Italian seasoning, Parmesan, egg + yolk, salt and pepper. Gently fold in the ground turkey, mixing just until combined — overmixing makes meatballs tough.
Cover or place the mixture on a tray and chill in the fridge for about 20–25 minutes so it firms up. Using slightly damp hands, roll into meatballs (about 1½ to 2 tablespoons each). Brush lightly with oil to help browning.
Heat oil in your skillet over medium-high heat. Sear meatballs in batches, turning so all sides get golden. Lower the heat to medium and let them finish cooking inside (internal temp ~165 °F / 74 °C). Remove and set aside.
In the same skillet, melt butter/ghee + olive oil. Sauté onions until soft, then add garlic and Italian seasoning. Whisk in pumpkin puree, stock, salt, pepper. Let simmer briefly, then stir in Parmesan, cream, maple syrup, and chopped sage off heat. Return the meatballs into the sauce and warm through for a few minutes so flavors meld.