In a large mixing bowl, whisk together the all-purpose flour and fine sea salt until combined. Create a well in the center, then add the two large eggs and whisk gently, incorporating the flour. Gradually pour in the milk while whisking continuously until smooth. Stir in the melted butter, cover, and let the batter rest for 30 minutes.

Heat a nonstick skillet over medium heat and brush lightly with melted butter. Pour approximately 60 ml of batter into the pan, swirling to form a thin layer. Cook for 1–2 minutes until the edges lift and bottom is golden. Flip and cook for another 30 seconds, then transfer to a warm plate. Repeat with the remaining batter.

Spread a thin layer of mustard on one half of a cooked crêpe, add ham and Gruyère cheese in the center, then fold into a triangle. Repeat with remaining crêpes.

Return the filled crêpes to the skillet over medium heat, pressing gently to encourage melting. Cook for 1–2 minutes on each side until cheese melts and edges become crispy. Serve warm, garnished with fresh parsley and black pepper.