01 - In a mixing bowl, whisk together flour and salt. Make a well in the center and add eggs and half the milk. Whisk until smooth, then gradually add remaining milk and melted butter. Batter should be slightly runny. Let rest for 10 minutes.
02 - Heat a non-stick skillet over medium heat and brush lightly with melted butter. Pour about ¼ cup batter into skillet, swirling to coat surface thinly and evenly. Cook for 1–2 minutes until edges lift and bottom is golden. Flip and cook 1 more minute. Transfer to a plate. Repeat with remaining batter.
03 - For the filling, heat olive oil in a skillet. Sauté onion until soft, then add mushrooms and cook until golden. Stir in spinach; cook just until wilted. Season with salt and pepper.
04 - Place 2 tablespoons filling and a sprinkle of cheese in each crepe. Fold or roll, then return to skillet to heat through and melt cheese, about 1–2 minutes per side. Serve immediately, garnished with fresh herbs if desired.