To make the crust: Fill a 1-cup liquid measuring cup with water and drop in a handful of ice cubes. Set aside.
In large, wide bowl, whisk the flour, sugar, and salt. Add the cold cubed unsalted butter and toss in the flour mix.
With a pastry blender, work the cubed butter into the flour mixture until the mixture becomes coarse and all the butter pieces are the size of tiny peas. Its okay if some are bigger than others.
Drizzle ½ cup of the ice-cold water over the flour mixture and with a plastic bench scraper (or sturdy silicone spatula), gather the dough together. The butter and flour mixture may need an additional ¼ cup of cold water, but add it 1 tablespoon at a time. Not all the water needs to be used. The butter and flour mixture should be a shaggy but clump together.
Turn the dough onto itself and divide in half. We’re making a double-crust pie! Wrap in plastic wrap to chill in the freezer for 30 minutes while you prepare the filling.
To make the filling: Preheat the oven to 425°F/218°C. Cut the butternut squash in half and reserve half for another dish. Peel and remove the seeds from the remaining half, and dice it into 1” cubes. Toss with 1 tablespoon of the olive oil, the garlic salt, and the chili flakes. Place on an oiled sheet tray and avoid over-crowding. Roast for 35 minutes, flipping at the halfway point.
You’re going to use one large sauté pan for the entire filling and gravy. Remember to cook on medium to medium-low heat so as not to cause any burning on the bottom of the pan. If you do, you can always deglaze the pan with a splash of water or chicken broth. Remove the casings from the lamb sausages. Heat 1 teaspoon olive oil in a large sauté pan over medium heat and cook the sausage, breaking it with a spatula into chunks until cooked through. Transfer the cooked sausage to a large bowl, leaving the drippings in the pan.
Add the shallot and onion to the sauté pan with the sausage drippings. Cook on medium heat until translucent, stirring them so they don’t burn.
Add the minced garlic and thinly sliced leeks and sauté until limp, 2 to 3 minutes. Season with salt and pepper and add the dried sage. Transfer the filling from the pan to the bowl with the cooked lamb sausage.
Dice the Granny Smith apple into ¾” cubes. Add the remaining 1 teaspoon olive oil to the same pan. Add the diced apple and sauté over medium-low heat until just soft and tender but not mushy, about 2 minutes.
Add the kale to the apples and continue to sauté until the kale wilts. Transfer the apples and kale to the bowl with the sausage mixture.
For the Mushroom Gouda Gravy: In the same pan, melt 2 tablespoons of the butter over medium heat. Add the mushrooms and cook until the water is released. Move the mushroom to the edges of the pan and melt the remaining 1 tablespoon butter in the center of the pan. Add the flour and cook for 1 minute.
Whisk in the chicken broth and milk slowly to avoid clumping and cook until thickened, 2 to 3 minutes. Season with salt and pepper.
With a wooden spoon, stir in the gouda cheese in a figure-eight motion until melted, about 2 minutes, and remove from the heat.
Pour three-quarters of the filling into the mushroom-cheese sauce and toss to coat all the filling. The sauce will not be too runny.
Remove the pie dough from the freezer and flour your work surface to start rolling out the bottom pie crust. Roll it a few times in one direction, lift, and rotate it a quarter-turn. Continue rolling a couple times, lift the dough, and rotate it. Re-flour the work surface and the top of the dough as needed. There should be no bits of dough on the counter and none should be stuck to the rolling pin. Roll the crust to 12” in order to fit a a 9” pie pan with 1” overhang.
Transfer the crust to a pie pan and wait to crimp the edges since we’re doing a double-crust pie.
Pour as much filling as will fit into the pie pan and transfer the entire pie pan to the refrigerator.
Roll out the second crust to 10” using the roll, rotate, turn method. This will be the top crust of the pie.
Remove the pie from the refrigerator. Place the top crust over the pie and pinch the edges together. Trim to just 1” overhang and fold the crust under. Crimp the edges until you have sealed the top and bottom together.
With the pie dough scraps you can get crafty by rerolling the dough and using cookie cutters to make fun shapes like leaves and acorns to decorate the top of the pie (using egg wash as glue). Make a few slashes in the pie to allow steam to escape during baking.
Return the pie to the refrigerator for 30 minutes to allow the crust to keep its shape before baking. I like to clean up and wash the dishes at this point. (Multitask win!)
After 30 minutes, preheat the oven to 425°F/218°C. Place the pie on a baking sheet, brush with the egg wash, and bake for 15 minutes. Then, lower the heat to 375°F/190°C and bake until bubbly, 40 to 50 minutes, checking halfway through that the crust is not burning. If it starts to brown, tent with foil.
Remove the pie from the oven and allow to rest for 10 minutes before cutting. It will be hot! This allows the gravy to settle into the filling, too. Slice and enjoy with a side salad.