Reduce the oven temperature to 375°F. Heat 2 tablespoons of bacon fat in a large pan and sauté diced onions, celery, and green peppers. Season with salt and pepper, and cook until the vegetables release their juices, about 5 minutes.
In a large bowl, combine the shredded turkey and sautéed vegetables. Add the sweet peas here if using.
Add 6 cups of crumbled cornbread, the stuffing mix, and 4 cups of turkey stock. Stir well.
Mix in the sage, poultry seasoning, and melted butter, followed by the remaining 4 cups of stock. Stir until fully combined. The dressing should look moist and loose, not dry or crumbly.
Spray two half-sheet pans with cooking spray and divide the dressing evenly between the pans.
Bake uncovered for 30 minutes, rotate the pans, then bake for another 30 minutes. The dressing is done when the top is well browned, the edges are crisp, and the center is set enough to portion while the interior stays moist.