Make the crepes: Combine the flour and salt in a medium bowl and make a well in the center. Whisk the eggs and milk together, and pour into the well. Whisk the batter until smooth, then whisk in the melted butter.

Strain the batter through a fine-mesh sieve and, if making savory crepes, stir in the chives.

Heat an 8-inch nonstick crepe pan or skillet over medium heat until hot. Brush lightly with butter, then ladle about 3 tablespoons of the batter into the center of the skillet. Rotate the skillet in a circular motion to swirl the batter and coat the bottom of the pan evenly with the batter. (If you hear it sizzle in the pan, your heat is too high.) Cook until the crepe is just set, 30 seconds-1 minute; you shouldn’t have to flip it. Use a small offset spatula to gently loosen the sides of the crepe and gently slide it onto a wax or parchment paper-lined plate. Repeat with the remaining crepes, very lightly brushing the pan with butter as needed and layering the paper and crepes.

For the sweet cheese filling: Combine the farmer’s cheese, egg, egg yolk, honey, lemon zest, and salt in a bowl and mix until smooth.

For the potato-cheese filling: Place the potatoes in a medium saucepan, cover with 2-inches of cold water and bring to a boil over high heat. Reduce the heat to medium-low and simmer until very soft, about 15 minutes.

While the potatoes are cooking, melt 2 tablespoons of butter in large skillet over medium heat. Add the onion and season lightly with salt. Reduce the heat to medium-low and cook, stirring, until the onions are soft and golden, about 15 minutes. Remove from the heat and set aside.

When the potatoes are soft, drain them in a colander and return them to the pot. Add the remaining 4 tablespoons of butter, and season to taste with salt and the white pepper, if using. Mash with a potato masher or fork until soft and creamy, 1 to 2 minutes. Stir in the onions, cheese, and chives until combined. Cover and let the mixture rest until cool enough to handle.

Fill the blintzes: Place ¼ cup of filling on each crepe about 2-inches from one end, shaping it into a 3-x1-inch log. Fold the lower edge of the crepe over the filling, then fold the sides inwards and continue to roll the blintz, burrito-style. Repeat with the remaining crepes and filling.

Cook the blintzes: Heat 1 tablespoon of butter in a large nonstick skillet over medium heat. Working in batches, add the blintzes seam side-down and cook until deep golden brown on each side, flipping once, 2-3 minutes per side. Transfer to a platter and repeat with the remaining blintzes. Cover them loosely with foil to keep warm.

To serve the blintzes: Serve the sweet cheese blintzes with sour cream and jam; serve the potato-cheese blintzes with creamy mushroom sauce.