Preparation

• Heat the oven to 375°F.

Make the topping:

• Combine the flour, both sugars, salt, and cinnamon in a medium bowl. Rub in the butter with your fingertips until it’s well blended and the mixture crumbles coarsely; it should hold together when you pinch it. Add the hazelnuts. Refrigerate the topping until you’re ready to use it.

Make the crisp:

• Put the apples and blackberries in a large  bowl. Taste the fruit and sprinkle on the sugar as needed (use 2 Tbs. sugar for very ripe fruit, or up to 1/3 cup for tarter, less-ripe fruit).

• In a small dish, dissolve the cornstarch in the lemon juice. Gently toss the mixture with the fruit.
• Pour the fruit into an 8- or 9-inch square (or similar-capacity) glass or ceramic baking dish. Set the pan on a baking sheet to catch overflowing juices. Top the fruit with half of the topping (keep the other half refrigerated) and bake for 20 minutes.
• Sprinkle the remaining topping over the crisp and continue baking until the fruit is tender when pierced with a knife, the topping is crisp, and the juices are bubbling, another 25 to 35 minutes. Let cool for 20 to 30 minutes. Serve warm.