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