Preheat an oven to 400 F. Set your oven rack to be in the top 1/3 of the oven. Line a rimmed baking sheet with parchment paper.
Saute the veggies and herbs. In a large braiser or oven-safe skillet, melt the butter over medium heat. Once the butter is melted and foamy, add the diced onion, carrot, and celery. Cook, stirring occasionally, for 4-5 minutes, then add the minced garlic, sage, thyme, and marjoram. Continue to cook until the veggies are soft and starting to brown. Let the vegetables cool for 10 minutes.
Soak the breadcrumbs. In a large mixing bowl, combine the bread crumbs, beaten eggs, and heavy cream and stir well to combine. Let this mixture sit for 10 minutes.
Mix the meatballs. Add the slightly cooled vegetables to the breadcrumb mixture and mix in the dijon, paprika, salt, and pepper until everything is well mixed. Add the ground turkey and pork and mix everything together by hand until the meatball mixture is homogenous and the ingredients are evenly distributed.
Portion and roast the meatballs. Use a 1/3 cup to portion the meatball mixture out and roll them into smooth balls. Set the meatballs on the prepared baking sheet. Transfer the baking sheet to the oven for 10 minutes, then broil on high for 2 minutes, just until the meatballs are slightly browned.
Reduce the oven temperature to 300 F.
Braise the meatballs. Transfer the meatballs to the braiser that you sautéed the veggies in and pour the turkey stock over the top. Cover with a lid, but leave the lid cracked an inch. Transfer the meatballs to the oven for 45 minutes. Remove from the oven, let the meatballs rest for 5 minutes, then use a slotted spoon to transfer them to a plate or tray. For a smoother gravy, use a spoon to skim out any pieces of meat or veggies left in the broth.
Make the gravy. Add the white wine and dijon mustard to the stock left in the braiser and bring to a simmer over medium heat for 5 minutes. While the stock reduces, mix the softened butter and flour together into a paste. Whisk the butter/flour paste into the stock as it's simmering and whisk well so that there are no lumps. Simmer for 4-5 minutes, until the gravy thickens and then season to taste with salt and pepper. Return the meatballs to the gravy and toss to coat. Serve warm.