Prepare the crépes:
Combine flour, egg, milk, sugar and salt in a bowl; mix until well blended. Melt a dab of butter in a 7-inch crépe pan over medium heat. Add 1/4 cup batter and cook for about 2 minutes on each side. Remove from the pan and stack between sheets of waxed paper. Let the crépes cool before filling; otherwise, the filling will melt too quickly.
Prepare the mascarpone cream:
Combine all ingredients and beat until well blended.
Prepare the topping:
Preheat oven to 400º. Combine blueberries, cherries and sugar in a bowl and mix gently to blend. Spread on a rimmed sheet pan and roast 10 minutes.
Lay the crépes flat. Put 1/4 cup of mascarpone cream on each one and fold in half. Top with hot roasted fruit.
Makes 8 servings.