In a large bowl, whisk together the all-purpose flour and fine sea salt. Create a well in the center and crack in the large eggs, slowly incorporating the flour into the eggs for a smooth batter.

Gradually whisk in the whole milk until the batter is smooth and lump-free. Stir in the melted unsalted butter. Cover the bowl and let the batter rest for 30 minutes at room temperature to enhance texture.

Preheat your nonstick skillet over medium heat and brush it lightly with melted butter.

Pour about 60 ml of the rested batter into the skillet and quickly swirl to spread it evenly. Cook for 1-2 minutes until the edges lift and the underside is golden brown. Gently flip it and cook for another 30 seconds before transferring to a plate. Keep warm.

If desired, spread a thin layer of Dijon mustard on the cooked crêpe. Add the cooked ham and sprinkle generously with grated Gruyère cheese. Fold the crêpe into a triangle, encasing the filling.

Return the filled crêpes to the skillet over medium heat. Cook for 1-2 minutes per side, or until the cheese melts and the edges become crisp. Serve immediately, garnished with fresh parsley and a sprinkle of black pepper.