01 - Whisk together flour and salt in a mixing bowl. Create a well in the center, then add eggs and half the milk. Whisk until smooth before gradually incorporating remaining milk and melted butter. The batter should have a slightly runny consistency. Allow 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 over medium heat. Sauté chopped onion until softened, then add sliced mushrooms and cook until golden. Stir in chopped spinach and cook just until wilted. Season with salt and pepper to taste.
04 - Place 2 tablespoons of filling and a sprinkle of grated cheese onto each crepe. Fold or roll the crepe, then return to the skillet to heat through and melt the cheese for approximately 1–2 minutes per side.
05 - Serve immediately, garnished with fresh herbs if desired.