Prepare the leek by cutting it in half (lengthwise) and running it under the cold tap. Separate the layers but only enough to wash out the grit, otherwise leaving the leek intact. Once clean, go ahead and slice each half nice and thin.
Melt butter in a large, casserole style pot and add sliced leeks. Fry them gently on a medium heat until softened but not coloured stirring frequently approx 8-10 min.
Add chopped/minced garlic and fry for another minute or so stirring.
Add pasta and ham followed immediately by ham poaching liquid or water and dijon mustard. Season to taste with salt and pepper.
Give everything a good stir, increase the heat to high and bring it to boil.
Pop the lid on, lower the heat and simmer gently for 12 minutes.
When the time is up, take the lid off and give the pasta a good stir (it will be quite saucy). Add frozen peas and cook for a couple of minutes until defrosted.
Follow with grated gruyere cheese and mix well.
Serve with more cheese sprinkled over if desired and some freshly chopped parsley.