Combine flour and salt in a food processor bowl with steel blade and blend for 8 seconds (or whisk together in a large bowl). Cut in 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.
Blanch asparagus for 4-5 minutes in boiling water, cool in ice water, drain and set aside.
Melt butter in a small sauté pan over medium heat. Add curry and cayenne pepper; cook one minute. Sauté onion in curry butter until soft and most of the moisture has evaporated; 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 asparagus over the cheese. Pour egg mixture over the top. Bake 30-35 minutes or until custard is set and golden.