Melted butter or vegetable oil to coat pan, as needed
2 cups whole-milk ricotta cheese
8 ounces softened cream cheese
1/4 cup sugar
3 large eggs
1/4 teaspoon vanilla extract
1/4 teaspoon salt
4 tablespoons butter, or as needed, to fry filled crepes
Mixed Berry Sauce:
1 1/2 cups fresh or frozen raspberries
1 1/2 cups fresh or frozen sliced strawberries
1 cup fresh or frozen blueberries
3/4 to 1 cup sugar or to taste
1 to 2 tablespoons fresh lemon juice or to taste
To make the crepe batter:
In a large bowl, whisk together flour, sugar and salt. Make a well in center of flour mixture. In a separate medium bowl, blend milk, eggs, 1 tablespoon melted butter, and vanilla. Add milk mixture to flour mixture and stir by hand just until smooth. Let batter rest in refrigerator at least 1 hour and up to 12 hours before preparing crepes. Strain batter, if necessary, to remove lumps before preparing crepes.
To make the crepes: Heat a crepe pan or small skillet over medium-high heat. Brush with melted butter. Pour about 1/4 cup batter into pan, swirling and tilting to coat bottom of pan with batter. Cook until first side is set and has a little color, about 2 minutes. Adjust temperature, if necessary. Flip the crepe and cook the other side about 1 minute more or until done. Stack crepes to fill now, or refrigerate or freeze them and assemble later.
To make the cheese filling:
In a large bowl, combine cheeses and sugar, and beat with a wooden spoon until thoroughly blended. Add eggs, vanilla, and salt and stir until smooth.
To make the mixed berry sauce:
In a medium saucepan, combine 1 cup raspberries, 1 cup strawberries, 3/4 cup blueberries, 3/4 cup sugar and 1 tablespoon lemon juice, and bring to a simmer over medium heat until sugar has dissolved, about 10 minutes. Adjust sugar, if necessary, and cook until dissolved. Puree the berry mixture and return to the saucepan and bring to a simmer. Add additional lemon juice, if necessary. Add remaining 1/2 cup raspberries, 1/2 cup strawberries and 1/4 cup blueberries, and simmer until sauce is hot. Sauce can be served hot or cold.
To assemble the blintzes:
Spoon 2 to 3 tablespoons filling on lower third of each crepe. Fold bottom of crepe over filling, then fold each of the remaining sides over to make a little package. Repeat with remaining crepes. Melt 1 tablespoon of the butter in a skillet over medium-low heat. Arrange 6 blintzes at a time seam-side down in skillet and sauté until faintly brown and crisp, about 2 minutes. Turn blintzes and brown on second side, another 2 minutes. Repeat with remaining blintzes, adding another tablespoon of butter for each batch.