Preheat your oven to 350°.
Melt butter in a skillet and add the diced yellow onion, carrot and celery with a pinch of salt. Cook 8 to 10 minutes until the carrots and celery are tender and the onions soft and translucent.
Next add in minced garlic and poultry seasoning. Stir and cook 1 minute.
Sprinkle in the flour, stir and cook for 1 to 2 minutes.
While stirring, pour in turkey stock. Simmer until it starts to thicken.
Remove off of the heat and add in the leftover gravy. Stir until combined.
Then add in leftover turkey, frozen peas and sweet corn. Stir to combine.
Season with 3/4 teaspoon kosher salt and 1/4 teaspoon freshly ground black pepper or to taste.
Transfer the filling to an 8×11 baking dish and spread evenly.
Top with 3 cups stuffing and spread evenly.
And spread the mashed potatoes over top.
Top with the grated cheddar jack cheese and bake on the middle rack of your preheated oven for 30 to 35 minutes.
Switch on the broiler and broil for 2 to 3 minutes or until lightly golden in spots.
Sprinkle with minced parsley and serve immediately.