Preheat oven to 400 degrees and lightly grease a 1-½ quart baking dish. Peel fresh peaches with a vegetable peeler and slice into medium-size bowl. Add berries, then brown sugar, flour, and ginger; toss to coat. Stir in Grand Marnier, if using. Bake for about 10 minutes.
While fruit is heating up, combine dry ingredients of oatmeal biscuit mix in medium bowl. Add beaten egg, milk, and melted butter, stirring just until moist. Do not over mix! Spread the biscuit mix over the fruit. (Note: If you have extra biscuit dough, bake them by the spoonful separately for a wonderful snack!)
Sprinkle lightly with cinnamon sugar. Bake for about 15 minutes or until biscuits are lightly golden.
Serve warm with fresh whipped cream or vanilla ice cream.