Dice the pork shoulder into large chunks, and season well with the salt, pepper and Italian seasoning. Sauté the meat in a frying pan or skillet on medium to high heat. When the meat has browned on all sides, you can transfer it to the slow cooker. Leave any grease in the pan.
Slice the leeks, and slice the onion and crush garlic. Sauté these vegetables in the frying pan with any leftover grease from the pork. Transfer these to the slow cooker as soon as the onion is soft and translucent.
Dice the carrots and potatoes into bite-sized chunks. Transfer these, along with all remaining ingredients except the cornflour and peas, to the slow cooker. Stir to combine the ingredients, cover with a lid, and cook on high for 4-6 hours, or on low for 6-8 hours.
Add the cornflour to a small bowl, and add 1-2 tablespoons of cold water. Stir to form a paste. Add this to the slow cooker. Add the frozen peas. Stir to incorporate the ingredients evenly, and cover with a lid.
Leave the stew to cook for a further 30-45 minutes. The casserole will gradually thicken. You can increase the amount of cornstarch paste if you want to make it even thicker.
