We make some version of this smoke-scented crisp all summer long. We start with strawberries (and often rhubarb) in May. In late June we use peaches. In July and August we use local blueberries, blackberries, and raspberries. As summer segues into fall, we make crisps with apples and pears.
Step 1: Pick through the blueberries, removing any stems, leaves, or bruised berries. Wash as needed. Place the berries in a large nonreactive mixing bowl. Add 1/4 cup of the flour, the granulated sugar, lemon zest, and lemon juice and gently toss to mix. Spoon the blueberry mixture into the cast iron skillet.
Step 2: Place the gingersnap crumbs, brown sugar, and the remaining 1/2 cup of flour in a food processor fitted with the metal blade and process to a coarse powder. Add the butter and salt and pulse the processor until the mixture is coarse and crumbly. Do not overprocess. Spoon the topping over the blueberry filling.
Step 3: Set up the grill for indirect grilling and preheat to medium-high (400 degrees).
Step 4: When ready to cook, place the skillet with the blueberries in the center of the hot grate, away from the heat, and cover the grill. Toss the wood chips on the coals. Cook the crisp until the filling is bubbling and the topping is browned, about 40 minutes. Serve warm with vanilla ice cream.
Variation: For an outrageously delicious variation, replace half of the blueberries (3 cups) with diced ripe peaches.