Cut butter into 1/2" cubes, wrap in plastic wrap, place in freezer for at least 30 minutes. Once butter has chilled, combine dry ingredients for crust and cut butter into dry ingredients (using blender or pastry cutter) until the butter is broken down into pea-sized pieces.

Add water to the mixture and knead until the dough just comes together when pressed between your fingers. Gather into a disc, wrap disc in plastic wrap, and place in refrigerator for at least 20 minutes to chill. Once dough has chilled, preheat oven to 425 degrees and roll out dough to fill pie pan. Bake crust until light brown, about 10-15 minutes. Set aside to cool.

Heat oil in sauce pan over medium heat and stir in onions. Cook onions on medium heat until soft, about 5 minutes. Add mushrooms, garlic and a generous amount of salt and sauté until all the water released from mushrooms has evaporated, about 10-15 minutes. Stir in baby spinach and cook until heated through, about 1-3 minutes. The spinach will shrink up a lot during this step. Remove from heat and set aside.

Break sausage up into bit sized pieces and brown in skillet over medium heat until fully cooked, about 10-15 minutes. The internal temperature should reach at least 160 degrees F.

Layer the sausage and mushroom-onion-spinach mixture along the bottom of the crust, cover with cheese. Whisk eggs, milk, salt, and pepper together. Pour egg mixture into pie crust until pan is filled (you may have some egg mixture leftover).

At this point you proceed to the next step and bake the quiche or you can cover the quiche, place in refrigerator, and let sit for up to a day before baking.

Once you are ready to bake the quiche, preheat the oven to 350 degrees F and place quiche pan on a baking sheet (to avoid spilling in oven). Bake until filling appears firm, about 45-55 minutes. Remove from the oven and let sit for at least 10 minutes before serving. Enjoy!