To Make Crust: In a large bowl combine 3 cups flour, 3/4 cups white sugar, baking powder, and 1/8 teaspoon salt. Mix thoroughly, then cut in 2/3 cup butter or margarine with a pastry blender, until pieces are the size of small peas. Stir in eggs, vanilla extract, milk and lemon zest. Mix just until all ingredients are combined. Allow dough to rest in refrigerator.
To Make Streusel Topping: In a medium bowl mix 1/2 cup flour, brown sugar, 1/2 teaspoon cinnamon, 1/4 teaspoon salt, chopped nuts, and grated lemon zest. Work in butter or margarine with fingers until all ingredients are well combined. Set aside.
To Make Fruit Filling: Place pitted and sliced fruit in a large bowl. In a small bowl, mix remaining sugar, flour, cinnamon, and nutmeg until thoroughly combined. Pour over fruit and stir gently until all fruit is evenly coated.
Roll out pie crust and place in a 9 inch pie pan. Trim and flute edges, then pour in fruit filling. Evenly cover fruit with streusel topping and bake in preheated oven for 45 to 55 minutes. Serve warm, or at room temperature.