The topping should be kept cold until ready to use; refrigerate in an airtight container, up to 5 days.
- 1 bag (12 ounces) frozen mixed berries
- 1/4 cup plus 2 tablespoons all-purpose flour
- 1/4 cup packed light-brown sugar
- 5 tablespoons granulated sugar
- 1/4 teaspoon ground allspice
- 1/2 cup rolled oats
- 4 tablespoons cold unsalted butter, cut into small pieces
- 3 pounds (5 to 7) ripe pears, peeled and cut into 3/4-inch pieces
- 1 tablespoon freshly squeezed lemon juice
- Vanilla ice cream, or frozen yogurt (optional)
Preheat oven to 400 degrees. Spread frozen berries in a single layer on a paper towel-lined baking sheet; thaw 30 minutes at room temperature.
Make oatmeal topping: In a large bowl, mix together 1/4 cup flour, light-brown sugar, 2 tablespoons granulated sugar, allspice, and a pinch of salt. Stir in oats. Using a pastry blender, two knives, or your fingers, incorporate butter into flour mixture until large, moist clumps form. Refrigerate.
In a large bowl, combine pears with lemon juice, thawed berries, 3 tablespoons granulated sugar, and 2 tablespoons flour; toss well. Transfer to a shallow 2-quart baking dish. Sprinkle evenly with chilled topping.
Bake until fruit is tender and topping is golden, about 45 minutes. Cool at least 20 minutes. Serve with vanilla ice cream or frozen yogurt, if desired.