Make the mashed potato topping by peeling and chopping the potatoes into 1-inch cubes. Bring a large pot of salty water to a boil. Add the potatoes and garlic to the pot. Cook for 10 minutes or until a fork inserted into a potato cube enters easily.
Drain the potatoes in a colander, and then return them to the still warm pot. This will help dry them out. Mash the potatoes with a potato masher or fork. Add the melted butter and mix until incorporated. Add the cheese and repeat. Mix in the salt and pepper, then add the parsley and cream. Mix and adjust seasoning to taste. Preheat the oven to 400°F.
Make the filling by placing the ground lamb in a large saute pan. Brown the meat, stirring frequently, until cooked fully, about 10 minutes on medium heat. Pour the meat and any pan juices into a heatproof bowl.
Do not wipe out the pan. Place the oil, mushrooms, onions, carrots, and garlic in the pan. Cook the vegetables, stirring frequently until the onions start to look translucent and the mushrooms start to soften. Add back the cooked ground meat, along with any juices that have accumulated at the bottom of the bowl back into the pan.
Pour in the chicken stock then add the tomato paste and sprinkle the flour over the mixture. Add the marmite and the salt, pepper, ground thyme and corn. Cook on medium heat, stirring constantly, for 10 minutes, until the filling has thickened. Stir in the frozen peas to the hot filling.
Pour the entire filling into a 9 x 13 inch casserole dish or an oval 3-quart capacity dish. Spoon the mashed potatoes into a pastry bag fitted with a star tip, then pipe the mashed potatoes over the filling in strips. Or spoon the mashed potatoes over the filling and then use the back of the spoon to indent and create ridges in the mashed potatoes. You want to create as much surface area as possible so the mashed potatoes can crisp.
Place the baking dish on a rimmed baking sheet (to catch any spills) and then place in the oven. Bake for 25 to 30 minutes or until the filling is bubbling up around the edges and the top of the mashed potatoes are golden brown.
Let cool 10 minutes before serving warm.