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Peel the potatoes and cut into 1/2-inch dice. Place in a medium saucepan and cover with cold water. Set over high heat, cover and bring to a boil. Once boiling, reduce heat to simmer. Cook until potatoes are easily pierced with a fork. Place the half-and-half and butter into a microwave-safe container and heat in the microwave until warmed through, about 35 seconds. Drain the potatoes and then return to the saucepan. Mash the potatoes and then add the half and half, butter, rosemary, salt and pepper and whip smooth. Temper the yolk in a separate bowl, the fold into the potato mix. Preheat the oven to 400 degrees F. While the potatoes are cooking, prepare the filling. Place the canola oil into a 12-inch saute pan and set over medium high heat. Once the oil shimmers, add the onion and carrots and saute just until they begin to take on color, approximately 3 to 4 minutes. Add the garlic and stir to combine. Add the lamb, salt and pepper and cook until browned and cooked through, approximately 3 minutes. Sprinkle the meat with the flour and toss to coat, continuing to cook for another minute. Add the tomato paste, broth, Worcestershire, rosemary, thyme, and stir to combine. Bring to a boil, reduce the heat to low, cover and simmer slowly 10 to 12 minutes or until the sauce is thickened slightly. Add the corn and peas to the lamb mixture and spread evenly into a greased 11 by 7-inch glass baking dish. Top with the mashed potatoes, starting around the edges to create a seal to prevent the mixture from bubbling up and smooth with a rubber spatula. Sprinkle with cheese, salt and pepper, if desired. Place on sheet pan on the middle rack of the oven and bake for 25 minutes or just until the cheese and potatoes begin to brown. Remove to a cooling rack for at least 15 minutes before serving.
