Pour the milk, butter, eggs and salt in a blender. Mix until well combined. Add the flour. Mix for 15 to 20 seconds or until smooth. Let stand for 10 minutes.

Heat an 8-inch crêpe pan or a nonstick skillet over medium heat. Coat well with nonstick cooking spray. Pour 1/4 cup batter into the pan, swirl to fully cover the bottom of the pan. Cook for 1 minute or until crêpe begins to curl around the edges. Carefully flip and cook for an additional 30 seconds or until set. Transfer to a plate. Repeat with additional cooking spray and remaining batter. Layer cooked crêpes between pieces of wax paper to prevent sticking.

Lay a crêpe on a clean work surface. Arrange a few slices of apple on a quarter of the crêpe; top with shredded ham. Fold the crêpe in half to cover the filling and fold in half again to create a triangular shape. Repeat with remaining crêpes, apple and ham. Place on a parchment-lined baking sheet and hold in a warm oven until ready to serve or up to 30 minutes.

For the cheese sauce, Melt the butter in a saucepan set over medium heat. Stir in the flour until well coated. Cook, stirring constantly, for 2 minutes or until pale and smooth. Whisk in the milk, a splash at a time, until smooth; stir in the mustard, salt and pepper. Cook, stirring constantly, for 5 minutes or until thick enough to coat the back of the spoon. Remove from heat. Whisk in the cheese, a small handful at a time, until melted and smooth.

Place filled crêpes on each plate. Spoon warm cheese sauce over each serving and sprinkle with chives.