For the meatballs, melt 2 tablespoons butter in a small skillet over medium heat. Add the breadcrumbs, then season with celery salt, poultry seasoning and pepper; toast to golden. Remove from the heat and cool.
Toss the toasted breadcrumbs with parsley and moisten them with apple cider. Set aside.
Heat a skillet over medium-high heat with a drizzle of oil. Add the bacon and cook to crisp. Remove and add the apples, celery, onion, thyme, salt and pepper. Sauté until tender in the bacon drippings, 3-4 minutes; sprinkle with the brown sugar, then add the vinegar and stock and let the liquid evaporate. Remove from the heat and cool completely.
Pre-heat the oven to 375°F.
In a mixing bowl, season the ground turkey with salt and pepper and add the breadcrumbs, cooled apple mixture and egg yolks; mix to combine. Roll into 2-inch balls and arrange on a parchment-lined, rimmed baking sheet. Bake until golden and a little crispy, 20-25 minutes.
For the gravy, melt the butter in medium size saucepot over medium heat. Add the pepper, stir for 30 seconds, then whisk in the flour and let the mixture come to a bubble. Whisk in the stock and thicken; add the Worcestershire and maple syrup. Remove from the heat and whisk in the mustard; season with salt, to taste.
Serve the meatballs with toothpicks and a warm bowl of gravy for dipping.