To make the crust: Whisk together the dry ingredients. Work the butter into the dry ingredients, sprinkle in the water, and mix gently just until the dough comes together. Shape the dough into a disk, wrap it, and refrigerate it for at least 30 minutes., Remove the dough from the refrigerator and roll it into a 14" circle. Place it in a lightly greased a 9” deep-dish tart pan or a 9” removable bottom or springform pan, folding any excess dough inward to reinforce the edges., Prick the bottom of the dough, then place it in the freezer for 20 to 30 minutes while you preheat the oven to 425°F., To make the filling: Sauté the vegetables (mushrooms, peppers, and onion) and salt in olive oil over medium-high heat until soft and lightly browned and most of the liquid has evaporated, about 10 to 15 minutes. Set aside to cool., To assemble and bake the quiche: Line the chilled crust with foil or parchment paper and fill it with pie weights or dried beans. Place the crust on a baking sheet (to catch any butter drips from the bottom of the pan). Bake the crust for 16 to 20 minutes, or until it's lightly browned around the edges., Remove the crust from the oven and gently remove foil or parchment with the weights or beans. Set aside., Combine the eggs, milk, and Pizza Seasoning. Set aside., Sprinkle about 1/3 of the shredded cheeses over the crust. Add the vegetables, then slowly pour in the egg-milk mixture. Top with the remaining cheese., Bake the quiche on the baking sheet for 30 to 35 minutes, adding a pie crust shield or foil after about 20 minutes if necessary to prevent the edges from over-browning. The filling should be set and the edges golden. A digital thermometer inserted into the center of the quiche should read 165°F., Remove the quiche from the oven and let it rest for 15 to 20 minutes before slicing., Store any leftover quiche, covered, in the refrigerator for up to three days.