Preheat oven to 180 °C. Bring a pot of water to the boil and cook leeks for about 10 minutes. Drain and let cool a little. Wrap leeks two by two in the ham. Put rolled-up leeks in an oven dish. Melt butter in a pan and add flour. Mix well to a smooth paste. Add milk bit by bit and keep stirring to prevent clumps. Cook sauce until thickened while you keep stirring. Add half the cheese and mix well. Season with salt and pepper. Pour sauce over leek rolls. Sprinkle rest of cheese over and bake for about 20 minutes until cheese on top has melted.