Preheat oven to 350F (180C).
Parbake the crust for 20 minutes. Remove from oven.
Spread spinach and feta cheese evenly over the pie crust. Place the crust on a baking sheet (to prevent spills).
Beat together eggs and combine with evaporated milk, dry mustard, and tarragon.
Carefully pour egg mixture (custard) over the filling ingredients.
Bake in a preheated 350F (180C) for 35 to 40 minutes or until a knife inserted near the centre comes out clean. Let stand for 5 to 10 minutes before cutting into wedges to serve.