In a large pot, boil the sweet potatoes in salted water on the hob until tender. Drain and leave to steam for 5 minutes.
Transfer the sweet potatoes back to the pot, season with salt and pepper to taste and add in a little butter if you like before mashing to a smooth consistency.
Add the oil to a large non-stick pot or frying pan on a medium-high heat.
Add the garlic and the chicken and cook for a few minutes. Remove from the pan, cover and set aside whilst you cook the vegetables.
Add a little more oil if needed, and then add the onion and cook for a few minutes until translucent.
Add the leeks and carrots and cook for a further 5 minutes until they soften.
Cover with the stock, the mustard if using, and season well with black pepper.
Add 2 tsp plain flour or cornstarch to a small cup, pour in 2 tsp cold water. Stir this into the leek mix and mix well.
Bring back up to heat for a minute to cook out the flour and ensure the stock has thickened to a good consistency.
Transfer the cooked chicken back into the pan and mix through the leeks.
(Separate out ⅓ of the mix and set aside to cool - this will be used for a different dish another day).
Transfer the remaining ⅔ cooked chicken and leek mix into a deep ovenproof dish.
Spread the sweet potato mash evenly over the chicken and leeks, and fork the top before placing in a preheated oven at 190C/170C fan for 35 minutes until the top is golden and the pie is fully cooked through.
Boil the peas whilst the pie is in the oven, and when ready, serve with the pie at the table.