serves 4  for dessert (with leftovers for breakfast)
I've baked this in every possibly tart, cake, pie, and gratin pan. Just don't use a dish with a removable bottom or it will leak.

for tart dough:
1 recipe for basic tart dough (make sure dough is refrigerated for at least 3 hours)
for streusel:
4 tablespoons salted butter
1/3 cup brown sugar
1/3 cup flour (for streusel)
1/2 cup finely chopped Marcona almonds (or try blanched almonds, pine nuts, walnuts, hazelnuts)
for fruit (totaling about 6 cups when cut up):
3 large peaches and 3 large nectarines (or any combination of pluots, plums, apricots)
1 pint blackberries (or olallieberries)
1 pint blueberries 
2-6 tablespoons sugar (depending on sweetness of fruit and size of your sweet tooth)
juice/zest from 1/2 lemon
2 teaspoons vanilla extract
pinch salt
2 tablespoons flour (for fruit)
for egg wash:
1 egg
2 tablespoons heavy cream

Remove tart dough from freezer or fridge and let it soften a bit. Roll out tart dough (about 1/2" thick). With a pizza cutter or knife, cut into 1" strips or any desired shape and place flat on a sheet pan. Refrigerate until fruit is ready.

For the streusel. Melt butter. Take off heat and mix in brown sugar, flour, and nuts. Set aside.

Preheat oven to 350°F.

For the fruit. It's best to peel all stone fruit (though plum skins can add nice tartness and color) by placing fruit in boiling water for 15 seconds and then shocking them in ice water. Skins will (usually) slide right off. Cut into similar size wedges directly into your baking dish. Add berries. Sprinkle with sugar, lemon juice/zest, vanilla, salt, and flour. Stir gently to combine.

For egg wash. Combine egg and cream.

Assembly. Place strips of dough over mixed fruit so that the pieces are overlapping a bit. You want to leave some lovely little pockets for steam and oozing juices to escape. Paint dough with egg wash. Spread streusel all over the top of the tart dough.

Bake until golden brown and bubbling (about 45 minutes but check after 30). Poke the tart dough to make sure it's almost firm. Serve immediately with ice cream. Or room temperature with heavy cream.