01 - Whisk together flour and salt in a mixing bowl. Create a well in the center, adding eggs and half the milk. Whisk until smooth, then gradually incorporate remaining milk and melted butter. Batter should have a slightly runny consistency. Allow to rest for 10 minutes.
02 - Heat a non-stick skillet over medium heat, brushing lightly with melted butter. Pour approximately ¼ cup batter into the skillet, swirling quickly to coat the surface thinly and evenly. Cook for 1-2 minutes until edges begin to lift and the bottom turns golden brown. 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 and translucent. Add sliced mushrooms and cook until golden brown. 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, approximately 1-2 minutes per side.
05 - Serve immediately while hot, optionally garnished with fresh herbs of your choice.