01 - Whisk together flour and salt in a mixing bowl. Create a well in the center, add eggs and half the milk, whisking until smooth. Gradually incorporate remaining milk and melted butter until batter reaches slightly runny consistency. Allow batter 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 skillet, swirling to create a thin, even coating. Cook for 1–2 minutes until edges begin to lift and 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 onion until softened, approximately 3 minutes. Add mushrooms and cook until golden, about 5 minutes. Stir in spinach and cook just until wilted, 1–2 minutes. Season generously with salt and pepper.
04 - Place 2 tablespoons of filling and a sprinkle of cheese onto each crepe. Fold or roll crepes, then return to skillet over medium heat. Cook for 1–2 minutes per side to heat through and melt cheese. Serve immediately.