In the bowl of a food processor, add 1½ cups flour and ½ teaspoon kosher salt. Pulse a few times to mix.

Cut butter into cubes and add into the food processor. Pulse for 3 to 5 seconds at a time until the mixture looks like coarse sand.

In a small bowl or glass measuring cup, add 1 egg and 2 tablespoons plus 1 teaspoon cold water. Whisk together. Add egg mixture into the food processor, secure the lid and pulse for 3 to 5 seconds at a time, until dough takes shape. Be careful not to overmix the dough.

Transfer the dough onto a lightly floured counter. Use your hands to shape the dough into a disk, then roll it out until the dough is in a circle big enough to cover the bottom and sides of your 8-inch springform pan. It should be about 1/8-inch thick.

Lay the dough onto your springform pan and use your hands to gently press it along the bottom and sides of the pan. Smooth out wrinkles to be the same thickness around the entire edge of the pan. Use a paring knife to trim the excess dough.

Place the crust into the freezer while you prepare your filling, or for at least 30 minutes. Note: If you’re preparing the crust a day or more before, cover it tightly with plastic wrap before placing in the freezer.

Heat a large skillet over medium-high heat. Add 1 tablespoon butter and 1 tablespoon olive oil. Once heated, add sliced onions and reduce heat to medium. Cook, stirring frequently, for about 20 minutes, until onions just start to caramelize. Add in ½ teaspoon salt and ½ teaspoon black pepper and cook for another 5 minutes. Remove onions and set aside.

Add remaining 1 tablespoon olive oil to the skillet. Once heated, add sliced mushrooms and cook, stirring frequently, for about 6 minutes. Add ½ teaspoon kosher salt and continue cooking to release the water, another 5 to 7 minutes. Stir in 3 cups chopped broccoli and cook for 2-3 minutes, until broccoli just starts to cook through. Remove from heat and fold onions back into the skillet. Set cooked vegetables aside.

In a large bowl, add 9 eggs, 1½ cups whole milk and ½ cup heavy cream. Whisk together. Add 1 teaspoon kosher salt and ½ teaspoon black pepper, along with 1 cup chopped parsley. Whisk and set aside.

Remove the prepared crust from the freezer. Add 2 tablespoons Dijon mustard to the bottom of the crust and brush it evenly along the bottom. Top with about ½ of the shredded white cheddar.

Spoon in cooked vegetables and use a fork or spatula to layer evenly. Sprinkle on the rest of the shredded white cheddar, reserving a few tablespoons.

Carefully pour in the egg and milk mixture. Use a spatula, as needed to spread the mixture evenly. Top with remaining cheese and place on middle rack to bake at 375 degrees F for 30 minutes.

After 30 minutes, reduce oven to 350 degrees F and bake for another 30 minutes. At this point, the top is likely to be browning. Tent with aluminum foil and bake for an additional 10 to 15 minutes (70 to 75 minutes total baking time). The quiche is done when the center has reached at least 165 degrees F and a knife stuck in the middle comes out clean.

Remove from oven and let cool on a baking rack for an hour, up to 90 minutes, before removing springform pan and cutting. This will allow the quiche to continue to cook and set. Slice and serve.