01 - Whisk together flour and salt in a mixing bowl. Create a well in the center and add eggs with 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 1/4 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 the chopped onion until softened 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 with salt and pepper to taste.
04 - Place approximately 2 tablespoons of the filling mixture in the center of each crepe and sprinkle with grated cheese. Fold or roll the crepe around the filling. Return to the skillet for 1-2 minutes per side to heat through and melt the cheese completely.
05 - Serve immediately while hot. Garnish with fresh herbs such as parsley, chives, or dill if desired. Accompany with a crisp white wine such as Sauvignon Blanc for an authentic French experience.