Using medium-high heat, add 2 tablespoons of bacon fat to large pan along with diced onions, peppers, and celery. Season with 1 teaspoon of salt and ½ teaspoon of black pepper. Cook just until vegetables release their juices, about 5 minutes. You're not trying to cook them fully, you just want them to release their liquids. This step can also be done well in advance and refrigerated until ready to bake.
Preheat oven to 375 F. In a very large mixing bowl, add shredded turkey, sautéed veggies, and peas - mix well to combine.
Add crumbled cornbread and stuffing mix to bowl, along with 4 cups of the turkey stock, and mix well to combine.
Then add poultry seasoning, sage, and melted butter to bowl and mix again.
Finally, add remaining 4 cups of turkey stock and mix one last time. I know it seems like a lot of mixing, but you really want everything evenly distributed in every bite.
Spray two half size sheet pans (18"x13") with nonstick cooking spray. Pour dressing mixture into pans, divided as evenly as possible.
Place pans on the middle and lower racks of oven and bake for 30 minutes. Then rotate the pans, bring the pan on the bottom up (and vice versa), and bake for another 30 minutes or so. It's almost impossible (within reason) to overcook this, so you may way to leave it in longer if you prefer crispier edges like my family.