Mix together the ricotta, mascarpone, sugar, vanilla extract, lemon zest and a small pinch of salt.
With the cooked side in, place the "tail" of the crepe toward you and spoon about 2 ounces of the filling inside the center of the wrapper. Fold and hold the two opposite sides of the crepe over the filling until they almost meet and then fold the "tail" over the ingredients pulling the stuffing toward you. Fold the crepe over until you form a seam on the blintz. Set finished 2-inch by 1 1/2-inch flat "eggroll" shaped blintz on a dry towel with the seam side down.
Melt butter or margarine in a skillet over medium high heat, beginning with the seam side down. Saute each blintz until golden brown, carefully so the cheese does not escape. Flip the blintz onto the other side. To warm the ingredients in the blintz, lower the flame and cook slowly, about 2 to 5 minutes to avoid burning the crepe.
Gently toss the rhubarb, strawberries, flour, sugar and lemon and let stand until moist, 5 to 10 minutes. In a large saucepan, melt the butter and add the fruit mixture. Cook the fruit for approximately 35 minutes, stirring to make sure the fruit does not brown. Cover 5 minutes or until rhubarb is soft. Ladle the warm fruit over the blintz and top with a dollop of whipped cream. Garnish with a fresh strawberry half.