Heat a large pot of water to a boil and boil the sweet potatoes whole until a fork easily passes through each one. When they're done, remove the potatoes and allow them to cool enough to handle. Peel the sweet potatoes then run them through a potato ricer or food mill to mash. Add the cheese and salt and pepper to taste.

For the gravy, melt the butter over medium low heat in a small saucepan. Add the flour and cook for about a minute, stirring constantly. Pour the stock and cognac in at once and whisk until smooth. Remove from heat and set aside.

For the lamb, heat a pan over medium heat and add a splash of olive oil. Add the onions and fry until translucent and fragrant. Add the lamb and use a spatula to break it up while frying until the meat is cooked. Add the mint, vinegar, sage, salt and pepper, then pour over the gravy. Cook over medium low heat stirring occasionally until the sauce has reduced a little and has become thick, about 15 minutes. Remove from heat and allow to cool. Stir in the peas.

Fill 5 ramekins ¾ full with the filling, then spread the sweet potato mixture on top. At this point you can cover and refrigerate (or freeze) the pies until you are ready to eat them.

Preheat oven to 400 degrees F. Place the ramekins on a baking sheet and put in the oven until the top is lightly browned.