Prepare a 13x9x2 inch casserole dish by greasing with about 1 tbsp of butter. Set aside.
Preheat the oven to 325°. Line a large baking sheet with parchment paper and spread bread cubes in an even layer. Bake until dry and toasted, about 15-20 minutes. Set aside.
In a large heavy bottom skillet, place sausage in the pan and over cook over medium-high heat, breaking up the sausage with a wooden spoon or spatula. No oil is needed, the sausage will begin to render its fat as it cooks. Cook until the sausage is a deep golden brown, not just “no longer pink.” You want flavor here, and color means flavor. Add garlic and red chili flakes and cook for about 1 minute. Using a slotted spoon, transfer to a large mixing bowl, add toasted bread and mix.
In the skillet you should have about 1 tbsp of fat from the sausage. To this, add 1 tbsp of olive oil and bring the heat to medium. Add onions, celery, ½ tsp of kosher salt and ¼ tsp freshly ground black pepper. Saute until the onions are slightly golden and the celery is soft, but still has a slight crunch to it, about 8 minutes. Add the dry sherry to deglaze and get all the last bits of sausage off the bottom of the pan. Add to the bread mixture and toss thoroughly. The bread cubes enjoy soaking up each of these layers of flavor.
To the now empty pan, add ¼ cup of butter and heat over medium heat. Once melted, add the parsnips and a ½ tsp of kosher salt and saute, stirring occasionally. After about 4-5 minutes, add the apples to the pan and saute together until tender and golden, about 4-5 additional minutes. If the mixture is sticking, the bottom of the pan is browning or you don’t hear a sizzle, you need more fat. Another tbsp of butter or splash of olive oil will help this. You can lower the heat to medium-low as well. Add to the bread mixture and combine.
Finally, add 2 tbsp unsalted butter to the pan over medium heat. Once melted, add chopped fresh sage and saute, stirring often for about 2 minutes. You should smell toasty butter and the sage should be dark green. Add to the bread mixture, mix thoroughly. Taste and see if you need one last pinch or salt or black pepper. We layered our seasoning with each step so this adjustment is rarely needed!
Add stuffing to the prepared casserole dish. Spread out evenly and drizzle with chicken stock. Cover with foil and bake for about 1 hour, removing the foil in the last 10 minutes or so to allow for the top to crisp up and brown. Serve warm.