For the filling:
3 large nectarines
4 large white-fleshed peaches (or regular peaches)
1 lemon, zested and juiced
3 Tbsp unbleached all-purpose flour
1/4 cup tightly packed light brown sugar
2 Tbsp granulated sugar
1/2 tsp pure vanilla extract
1/2 cup golden raisins or dried cranberries
Pinch of kosher salt
For the topping:
1-1/4 cups all-purpose flour
1/2 cup rolled oats
1/2 cup light brown sugar
Scant 1/2 cup granulated sugar
1-1/4 sticks (10 Tbsp) cold unsalted butter, cut into pea-sized pieces
1/2 cup sliced almonds (optional, but delicious)
Pinch of kosher salt
1-2 Tbsp cold water
Preheat the oven to 350°F, and set aside a gratin dish approximately 9x11 inches and at least 2 inches deep.
Pit the nectarines and peaches, and cut into bite-size chunks. Toss them in a mixing bowl with the lemon zest and juice. Add the rest of the filling ingredients, and stir to combine. Pour the filling into the gratin dish and spread evenly.
Combine all of the topping ingredients, except the water, in a food processor. Pulse until combined (approximately 20-30 seconds). Add water, 1 tablespoon at a time, until the mixture is slightly clumpy, and still crumbly.
Sprinkle the topping over the fruit; don't pack it down, just sprinkle lightly. You might not use all of the topping, depending on the size of your gratin dish (I use 2/3 of the topping, and save the rest in the refrigerator for another small crisp). There should be bits of fruit peeking out here and there.
Place the dish into the preheated oven and bake for 30-32 minutes, until the filling is bubbling around the edges and the topping is lightly browned and crisp.
Serve hot or warm, topped (if you wish) with vanilla ice cream, frozen yogurt, or Greek yogurt.