Preheat the oven to 375 degrees F.

Roll the premade pie crust out into a pie pan. Crumble up parchment paper and place over the pie crust. Fill with pie weights or dry beans.

Place pie crust in the oven and bake for 10 minutes. Remove the parchment paper and pie weights and set aside. Bake the crust for 5 more minutes.

Heat the butter in a heavy bottom skillet on the stovetop over medium heat. Add the onion and mushrooms, cook till mushrooms start to lose their juices, about 10 minutes.

Add the garlic, thyme, 1/2 tsp sea salt, and black pepper. Stir to combine, remove from the heat when garlic becomes fragrant, about 2 minutes.

Thoroughly combine the eggs, milk, and remaining 3/4 tsp sea salt in a bowl.

Add the onion mushroom mixture to the pie crust, top with the cheese and pour egg mixture over the filling.

Bake the quiche for 35 to40 minutes until the crust is browned and the filling is set, and the center has a light jiggle.

Let the quiche cool at least 5 minutes. Serve warm.