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1
Preheat an oven to 365 degrees F (185 degrees C). Lightly oil a 9-inch square casserole dish.
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2
Sift the 3 cups flour, cinnamon, and nutmeg together into a large mixing bowl. Cut the vegetable shortening and cold butter into the flour mixture with a knife or pastry blender until the mixture resembles coarse crumbs with some chunks the size of peas. Add the ice water a tablespoon at a time, tossing with a fork, until the flour mixture is moistened. Work the dough as little as possible; this is the key to a super light and flaky crust. After half the water has been incorporated, watch closely, as mixture will come together suddenly. Once it begins to stick together, gather into a ball, divide into halves, and roll each ball out to 1/4-inch thickness, adding flour as necessary to prevent sticking. Press one of the dough portions into the prepared casserole dish, covering bottom and sides completely.
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3
Heat a skillet over medium heat. Sear the chicken breasts in the hot skillet until evenly browned on both sides; cut into small cubes and set aside in a large mixing bowl. Reduce the heat under the skillet to medium-low. Cook the carrots, celery, potatoes, peas, and about half the diced onion in the drippings from the chicken until the carrots soften; add to the bowl with the chicken.
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4
Increase the heat to medium. Add the water to the skillet and bring to a boil while scraping the browned bits of food off of the bottom of the pan with a wooden spoon; pour into a cup and set aside.
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5
Melt the butter in the skillet. Cook and stir the remaining onion and the garlic in the melted butter until the onions begin to brown, 5 to 8 minutes. Sprinkle the 2 tablespoons flour over the onion mixture and stir to evenly coat. Pour the reserved liquid over the onion mixture. Stir the chicken base, salt, pepper, sage, and thyme into the mixture. Cook and stir until the mixture has reduced into a thick gravy; pour over the chicken mixture in the mixing bowl. Allow the mixture to cool completely.
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6
Pour the mixture over the crust in the casserole dish. Top with the remaining portion of dough and pinch the edges together to seal the edges. Cut a few slits into the top to vent.
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7
Bake in the preheated oven until the crust is golden brown and the gravy bubbles through the slits, about 1 hour. Allow to cool slightly before serving, 10 to 15 minutes.
