Plum-Nectarine Crumble With Hazelnuts

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Unsalted butter, for the baking dish

2 pounds nectarines (about 5), sliced 1/2 inch thick

1 pound plums (about 4), sliced 1/2 inch thick

3/4 cup granulated sugar

2 tablespoons all-purpose flour

1 tablespoon fresh lemon juice

1/4 teaspoon salt

1/2 cup all-purpose flour

1/2 cup whole-wheat flour

1/2 cup packed light brown sugar

1/2 teaspoon ground cinnamon

1/2 teaspoon salt

2/3 cup blanched hazelnuts, chopped

1 stick unsalted butter, at room temperature

Vanilla ice cream or whipped cream, for serving