01 - Whisk together flour and salt in a mixing bowl. Create a well in the center and add eggs with half the milk. Whisk until smooth, then gradually incorporate remaining milk and melted butter. The batter should have a slightly runny consistency. Allow it to rest for 10 minutes.
02 - Heat a non-stick skillet over medium heat and lightly brush with melted butter. Pour approximately ¼ cup batter into the skillet, swirling to coat the surface thinly and evenly. Cook for 1-2 minutes until edges begin to lift and the bottom turns golden. Flip and cook for 1 additional minute. Transfer to a plate and repeat with remaining batter.
03 - Heat olive oil in a skillet. Sauté the onion until softened, then add mushrooms and cook until golden. Stir in spinach and cook just until wilted. Season with salt and pepper to taste.
04 - Place 2 tablespoons of filling and a sprinkle of cheese onto each crepe. Fold or roll the crepe, then return to the skillet to heat through and melt the cheese, approximately 1-2 minutes per side. Serve immediately, garnished with fresh herbs if desired.