In a large bowl, whisk together flour and salt, then create a well in the center and add eggs. Gradually whisk in milk until batter is smooth.
Stir in melted butter, cover the bowl, and let the batter rest at room temperature for 30 minutes.
Heat a nonstick skillet over medium heat for 2-3 minutes and lightly brush with melted butter.
Pour about 1/4 cup (60 ml) of batter into the center of the hot pan and swirl to spread thinly and evenly.
Cook for 1-2 minutes until edges lift and bottom is golden, then flip and cook for 30 seconds more. Transfer to a plate and keep warm.
Repeat cooking remaining batter, brushing pan with butter as needed.
Place one crêpe flat, optionally spread Dijon mustard down the center, then layer with ham slice and sprinkle with Gruyère cheese.
Fold crêpe in half, then in half again to form a triangle enclosing the filling.
Return filled crêpes to the skillet over medium heat, press gently, and cook 1-2 minutes per side until cheese melts and edges crisp.
Serve immediately garnished with chopped parsley and freshly ground black pepper, if desired.