In a large mixing bowl, whisk together half of the milk, the eggs, sugar, salt, and flour until smooth. Stir in the vegetable oil, then gradually mix in the remaining milk until the batter is silky. Let it rest for about 10 minutes.

While the batter is resting, heat a skillet over medium-low heat and add a drizzle of vegetable oil. Sauté the chopped onion until translucent, about 4-5 minutes. Next, add the chopped mushrooms and cook for another 8-10 minutes. Season and stir in sour cream for a creamy filling.

Heat a lightly greased non-stick pan over medium heat. Pour in a ladle of batter to create a thin pancake, cooking for about 3 minutes on one side until lightly browned. Flip the pancake and cook for another minute until golden. Remove and stack on a plate.

For each pancake, lay it flat, spoon about 2 tablespoons of filling onto one half, and fold it over. Repeat for remaining pancakes.

Serve warm or cold, topped with sour cream and fresh herbs. Enjoy these delightful pancakes anytime!