1. Preheat oven to 250°. Line a baking sheet with parchment paper, and set aside.
2. Beat egg whites and cream of tartar at high speed with an electric mixer until very foamy. Add sugar, about 1 tablespoon at a time, beating 2 to 4 minutes or until stiff peaks form and sugar dissolves.
3. Drop meringue onto prepared pan. Gently spread meringue with a rubber spatula into a 10-inch round on prepared baking sheet. Bake 1 1/2 hours. Turn heat off, and let cool in oven about 2 hours. Remove from oven; cool completely.
4. Loosen meringue from parchment paper with a spatula or knife, and transfer to a plate or cake stand. Stir together mascarpone cheese and next 3 ingredients. Spoon cheese mixture onto meringue; top with orange segments and berries.
*If mascarpone or cream cheese is thick, thin with 1 to 2 tablespoons fresh orange juice.