Topping:

1/2 cup (65 grams) all purpose flour

1/4 cup (50 grams) granulated white sugar

1/4 cup (55 grams) light brown sugar

1/2 teaspoon ground cinnamon

1/4 teaspoon fresh or ground nutmeg (optional)

1/8 teaspoon salt

6 tablespoons (84 grams) cold unsalted butter, cut into pieces

1/3 cup (30 grams) old-fashioned rolled oats

1/3 cup (40 grams) chopped walnuts or pecans

Filling:

6 cups (1.2L) Granny Smith Apples or other firm, tart-tasting apple (peeled, cored, and cut into 1 inch (2.5 cm) chunks)

1 cup (240 ml) fresh blackberries or raspberries (optional)

Juice of 1/2 lemon

1 teaspoon lemon zest

3 tablespoons (40 grams) white granulated sugar