To the slow cooker pot, place in the ham and pour in chicken stock and cook on high for 5-6 hours.
When ready, transfer to a bowl and keep the stock for later.
Pull the ham apart with forks then set aside.
Add butter and oil to a pan on a medium high heat, then add sliced leeks.
Fry for 3-4 minutes until softened.
Then add wholegrain mustard and garlic, mixing through.
Add plain flour, stir through then add the stock from the ham followed by milk.
Stir and bring to a gentle simmer to thicken the sauce, then add fresh parsley.
Stir again, then add peas.
Stir those through, then add the shredded ham to the sauce, mixing through, then set aside.
Then, peel chop and par boil potatoes for 10-12 minutes.
When ready, add melted butter, mixed herbs, salt and pepper, mixing until fully coated.
Transfer the ham and leek filling to an ovenproof dish.
Then evenly place the potatoes on top.
Place in a preheated oven at 180c (fan) for 40-45 minutes or until the potatoes are golden on top.
Finally add a sprinkle of cheese on top and return to the oven for 5 more minutes until melted.
Garnish with fresh parsley and dig in