Cut the onion in half through the ends, chop celery pinto 2 or 3-inch pieces, slice the garlic head in half crossway.
Make a garnie bag with thyme, broken bay leaves, and peppercorns. Tie the parsley together with butcher twine.
Rinse the thighs and pat dry with a paper towel.
Place a large stockpot over medium-high heat
When hot, place chicken thighs in a stockpot, skin side down. Brown chicken for about 4 or 5 minutes, then turn over and add the turkey necks. Continue to brown the thighs and necks for about 5 minutes, turning the necks every couple of minutes.
Remove the thighs and necks, leaving the rendered chicken fat.
Place the onions and garlic into the stockpot, cut side down. Add the celery. Brown the vegetables for about 5 minutes leaving the onion and garlic cut side down.
Once browned, add the thighs, necks, parsley, and Garni bag.
Increase the heat to high. Add 8 cups of cold water, stir.
Bring the stockpot to a boil, then reduce the heat to a slight simmer. After 15 minutes, skim the impurities off the top.
Maintain a very low simmer, partially covered for about 1 hour (total), then remove the thighs and set them aside to cool.
Continue to low simmer the thighs for another 30 minutes. , then remove them and set them aside to cool.
Strain the liquid and discard the vegetables.
Once the thighs and necks are cool, remove the chicken skin and pull the meat off the bones. Strip the meat off the turkey necks. Set aside.
Place the chicken and turkey meat in a small food processor. Sprinkle a little Creole seasoning on top and grind until fine. May need to do this in batches. Set aside. (See Notes)
Make the dressing
Place a Dutch oven over medium-high heat and add the vegetable oil.
When hot, add the yellow onions, celery, and green onions and fry until the yellow onions are translucent, 10 minutes.
Add the ground chicken and turkey meat. Mix well and cook for 10 to 15 minutes, stirring occasionally.
Add the Herb and Spice Blend (except the sage)
While the vegetables and chicken meat are simmering, add about 2 to 3 cups of oyster juice to the bread cubes. If you don't have enough oyster juice, use some of the reserved stock. Mix well and set aside, stirring once or twice.
After the poultry meat has simmered with the vegetables for about 15 minutes, add the raw oysters and the parsley. Stir to mix completely.
Oysters will release liquid as they cook, so use that to deglaze the pan, if needed.
Cook until the oysters curl up and the liquid reduces. This should take about 5 to 10 minutes.
Add about 6 cups of the soaked bread cubes to the vegetables and oysters and mix well to combine.
Add the ground sage and mix thoroughly. Add the remaining oyster juice (if any) or stock and more bread if needed. Remain on the heat for about 5 minutes.
Remove the Dutch oven from the heat and cover. Let sit for about 10 minutes.
Bake the dressing
Prepare a baking dish with non-stick spray, oil, or softened butter. Preheat the oven to 350 degrees Fahrenheit.
Fold the oyster dressing into the prepared baking dish and spread evenly.
Place the baking dish in the preheated oven and bake for about 30 minutes or until the top is browned and bubbling around the edges.
Remove from the oven and set aside for about 10 minutes.
Notes
Use the stock that was made by cooking the thighs and necks, or substitute commercial chicken stock.If you don't have a food processor, you can use a meat grinder with a medium chopper plate or finely chop the mat with your chef's knife.Leftovers can be held in the refrigerator for 3 to 4 days in an airtight container. Reheat to 165 degrees Fahrenheit in the oven, on the stove, or in the microwave.You can make this recipe in advance, up to the point of baking. However, the USDA recommends never to refrigerate uncooked dressing due to the risk of bacterial growth. Instead, you can freeze the uncooked dressing. Make sure you have an airtight seal. The dressing can be baked from frozen. Follow the instructions for baking, but you may need to add a little time to reach 165 degrees Fahrenheit.This recipe can be made in total or in part a day or two ahead. You can prepare the thighs and necks the day before with the stock. If you make parts in advance, don't combine all the ingredients until you are ready to bake. See the above Note.