Quiche makes a comforting main dish for a family supper or a casual dinner party. Save time by making and baking the wholesome Basic Pie Pastry 24 to 48 hours ahead of time. You can even prepare the sautéed mixture in advance—just refrigerate until needed.
To Make Pie Pastry
1. Prepare Basic Pie Pastry, and set aside to rest 10 minutes. Grease 9&1/2-inch deep-dish pie pan. Preheat oven to 400°F.
2. Roll pastry into 12-inch circle between two sheets of wax paper or plastic wrap. Peel off top sheet; invert pastry over pie pan. Peel off second sheet, then tuck pastry into pan without stretching it, pinching overhang into an even edge. Chill 15 minutes. Smooth sheet of aluminum foil over pastry (it should fit like a glove); fill with dried beans or rice. Bake 20 minutes. Remove pan from oven; lift foil with beans from pan. Using fork, pierce pastry 6 or 8 times. Reduce oven to 375F, and bake 7 minutes. Cool on wire rack. Leave oven on.
To Make Filling
3. Heat oil in large skillet over medium heat. Add onion, and cook, stirring, 5 minutes. Add mushrooms; cover, and cook 3 to 4 minutes, stirring occasionally. Stir in spinach and garlic; cover, and cook 2 to 3 minutes, or until spinach is wilted.
4. Lightly whisk eggs in mixing bowl. Whisk in milk, mustard, pepper and salt.
5. Sprinkle 1/4 cup grated cheese over bottom of pastry. Bake 2 to 3 minutes to melt cheese.
6. Spread vegetable mixture in pastry shell. Sprinkle with half of remaining cheese. Slowly pour in egg mixture, then sprinkle with remaining cheese. Bake 40 to 45 minutes, or until golden brown and slightly puffy. Transfer to wire rack, and let cool 15 minutes before serving.