Mix buckwheat flour with salt, milk, water, and eggs together. Add melted butter and whisk until you get a smooth batter. Cover with plastic and and let rest for at least 2 hours. When you are ready to cook the crepes, stir the batter before frying each crepe.
In a medium skillet, heat the oil add onion and sauté for 3-5 minutes.
Add sliced mushrooms and sauté for another 5 minutes.
Add the sour cream, mix well, and cook for another few minutes. Season with salt and pepper. Remove from heat.
Heat a lightly buttered nonstick skillet. Add 1/4 cupful batter to skillet and immediately tilt to fully cover the bottom of the pan with a thin layer. Cook for a few seconds until the surface isn't liquid.
Without flipping the crepe, place 2-3 Tbsps of mushroom's filling in the center. A 10-15 seconds later, as the crepe seems to be ready, carefully lift the sides with a spatula and fold them to form a square, or triangle.
Carrefully transfer the crepe from the pan to tha plate, and serve quickly.