Spread frozen berries in a single layer on a paper towel-lined baking sheet; thaw for 30 minutes at room temperature.
While berries are thawing, make oatmeal topping: Mix together in a large bowl flour, sugars, allspice and salt; stir in rolled oats. Blend butter into flour mixture with a pastry cutter until large, moist clumps form.
Returning to the crisp -- prepare pears by peeling and cuting into 3/4" pieces. Combine pears in a large bowl with lemon juice, thawed berries, sugar and flour; toss well.
Transfer pear/berry mixture to a 2-quart baking dish spritzed with baking spray for easy clean-up.
Sprinkle evenly with chilled topping.
Bake until fruit is tender and topping is golden (about 45 minutes). Cool at least 20 minutes, and serve with vanilla ice cream or frozen yogurt (if desired).