Preheat oven to 375°.
Grease an 8" X 8" X 2" baking pan. Set aside.
In large mixing bowl, gently combine sugar, flour, and cinnamon together.
Add mixed berries and gently combine.
Stir in lemon juice and lemon zest.
Spoon fruit mixture into prepared 8" X 8" X 2" pan.
Separately in small mixing bowl, stir together rolled oats, flour and brown sugar.
Stir in melted butter with a fork until crumbly.
Sprinkle oatmeal topping lightly over filling in baking pan.
Bake uncovered for 45 minutes or until golden brown.
Serve warm with a splash of fresh cream or scoop of vanilla bean ice cream.