1. Combine 1 1/4 cups flour, 1/2 tsp. sugar, and 1/4 tsp. salt in a food processor. Add 1 stick of cold unsalted butter, cubed; pulse until pebbly. Add 3 Tbs. ice water and 1 tsp. lemon juice, combining until it just holds together. Remove and pat into a disc. Wrap in plastic (we like Reynolds Kitchens Quick Cut Plastic Wrap); chill for 1 hour. Roll dough onto parchment and place on a rimmed baking sheet.
2. In a large bowl, combine 3 cups fruit (peeled and chopped if necessary), 1 to 2 Tbs. sugar (depending on sweetness of fruit), 1/2 Tbs. cornstarch, 1 tsp. vanilla, juice of half a lemon, and 1/16 tsp. salt.
3. Preheat oven to 425 degrees F. Spoon filling into center of dough, leaving a 2-in. border. Fold edges of dough up and over filling, then brush with egg wash and sprinkle with sugar. Dot fruit with 1 Tbs. butter; bake 10 minutes. Reduce oven to 375 degrees F, and continue baking until pastry is golden and juices are bubbling, 20 to 25 minutes more. Let cool 30 minutes.