- Make the crust: Preheat oven to 375 degrees. Roll out dough into a circle, place into pie pan with a 1-inch overhang and crimp. Blind bake pastry—lay parchment over dough and, using dried beans or pie weights, bake for 20 minutes until edges are puffed and slightly golden. Remove weights, poke all over with a fork, and bake uncovered for an additional 10 to 15 minutes until bottom is baked through, but not dark. Let cool slightly. Once cool, spread mustard across bottom.
- Make the filling: In a large skillet, heat olive oil and add garlic, rosemary, and mushrooms. Add salt and pepper to taste. Cook on medium-high heat until liquid evaporates and mushrooms are caramelized. Let cool.
- Reduce oven temperature to 350 degrees. Whisk eggs, flour, cream, milk, grated cheese, nutmeg, salt, and pepper. Pour in pie shell. Add mushroom mixture in an even layer. Bake for 45 to 50 minutes, rotating quiche halfway through baking. The filling should rise slightly and brown on top. Let cool before cutting.