Make the Crepe batter: In a bowl combine the milk, 2 tablespoons of the melted butter and eggs. Add the flour, sugar and salt and use an electric mixer on low speed to blend the batter until smooth, about 1 minute. Let the batter stand for 45 minutes at room temperature.
Meanwhile, make the Filling: In a large bowl combine the cottage cheese, cream cheese, farmers cheese, sugar, egg, vanilla and lemon zest, if using, with an electric mixer until fairly smooth, about 1 minute. Keep in the fridge until ready to use.
Make the Blueberry Sauce: Combine the blueberries, ¼ cup sugar, cornstarch, and juice, in a small saucepan. Heat, stirring frequently, over medium heat until the sauce comes to a simmer. Use a potato masher or a fork to mash up some of the blueberries. Simmer for 1 to 2 minutes until thickened, and taste to see if you want to add more sugar. Set aside and cover to keep warm (if the sauce thickens too much you can stir in a tablespoon or two of hot water).
Heat a 10-inch skillet over medium heat and brush with a bit of the remaining melted butter. Scoop ¼ cup of crepe batter into the pan and quickly tilt and swirl the pan around so that the batter coats the bottom of the pan. Allow the bottom to set, about 30 seconds, then flip the crepe with a large spatula. Cook for another 30 seconds until the bottom is set and pale golden. Transfer to a plate. Repeat with the remaining melted butter and batter, stacking the crepes on top of each other.
Place 3 tablespoons of filling along the top middle of each crepe (don't be tempted to overstuff the crepes or the filling might seep out as you cook them). Fold up the top part over the filling, fold in the sides, then roll up the crepe so the filling is completely encased in a rectangular packet. Repeat with the rest of the filling and crepes.
Brush a little bit of butter in the skillet over medium high heat. Add as many of the filled crepes as you can fit without crowding. Fry the crepes until golden brown on each side, 2 to 3 minutes per side. Serve hot or warm with blueberry sauce.