01 - Whisk together flour and salt in a mixing bowl. Create a well in the center, add eggs and half the milk. Whisk until smooth, then gradually incorporate 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 quickly 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 the chopped onion until soft and translucent, approximately 3-4 minutes. Add sliced mushrooms and cook until golden brown, about 5 minutes. Stir in chopped spinach and cook just until wilted, about 2 minutes. Season generously with salt and pepper.
04 - Place 2 tablespoons of the filling mixture and a light sprinkle of cheese onto each crepe. Fold or roll the crepe around the filling. Return filled crepes to the skillet and cook for 1-2 minutes per side until heated through and cheese is melted. Serve immediately.