Combine flour and salt in a food processor bowl with steel blade and blend for 8 seconds
Cut in frozen butter by pulsing 8 times with food processor (or use a pastry cutter), until mixture resembles coarse meal. Do not over blend.
Add water and mix just until dough comes together. Put dough in a plastic bag and chill 30 minutes.
Roll on a lightly floured board and fit pastry into an 11-inch tart pan with removable bottom. Chill until ready to fill.
Melt butter in a small sauté-pan over medium heat. Sauté onion and zucchini in butter until tender; set aside to cool. In a small bowl beat, eggs, milk and salt.
Preheat oven to 375°F. Sprinkle cheese into chilled pastry shell. Spread onion mixture and zucchini over the cheese. Pour egg mixture over the top. Bake 30-35 minutes or until custard is set and golden.
Note: The secret to a perfectly cooked quiche is removing it from heat before it becomes too firm or dry. It will continue to set as it cools. Serve warm or cool. Refrigerate any left-overs.