6 cups assorted organic berries (fresh or frozen, such as blueberries, blackberries or hulled strawberries)
  Grated zest of 1 orange and 1 lemon
1 tablespoon fresh lemon juice
2 tablespoons orange-flavored liqueur, such as Cointreau or Grand Marnier (optional)
1/2 cup granulated sugar
2 tablespoons cornstarch
3/4 cup unbleached all-purpose flour
1/2 teaspoon allspice
1/4 teaspoon salt
1/2 cup firmly packed dark brown sugar
2 tablespoons granulated sugar
6 tablespoons unsalted butter
1/2 cup old-fashioned rolled oats (not instant)
1 cup crème fraîche (for serving, optional)