1.5 pounds russet potatoes
1/4 cup milk
2 oz. butter
1 teaspoon kosher salt
1/2 teaspoon freshly ground black pepper
1 egg yolk (optional)
1/4 cup shredded cheddar cheese (optional)
2 Tbsp canola oil
1 cup chopped onion
2 carrots, peeled and diced small
3 cloves garlic, minced
3/4 pound ground lamb
3/4 pound ground beef
1 tsp kosher salt
1 tsp freshly ground black pepper
2 Tbsp all-purpose flour
1 Tbsp tomato paste
1 cup chicken broth
2 tsp Worcestershire sauce
1 Tbsp freshly chopped rosemary leaves
1 tsp freshly chopped thyme leaves
1/2 cup fresh or frozen corn kernels
1/2 cup fresh or frozen English peas
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, uncover, decrease the heat to maintain a simmer and cook until tender and easily crushed with tongs, approximately 10 to 15 minutes. Place the milk and butter into a microwave-safe container and heat in the microwave until warmed through, about 35 seconds. Drain the potatoes in a colander and then return to the saucepan. Mash the potatoes and then add the milk, butter, salt and pepper and continue to mash until smooth. Stir in the yolk until well combined.
Preheat the oven to 400 degrees F.
While the potatoes are cooking, prepare the filling. Heat 1 Tbsp of oil in a 12-inch saute pan over medium high heat. Add the meat and cook until thoroughly browned. Place in a colander and drain fat. Return pan to heat and add 1 Tbsp of oil. Saute the onion and carrot until they begin to take on color, about 3 to 4 minutes. Add the garlic and cook 30 seconds to a minute, season vegetables with salt and pepper. Return the browned meat to the pan then sprinkle with flour and toss to coat, cook for another 1 minute. Add tomato paste, broth, Worcestershire, rosemary, thyme, and stir to combine. Bring to a boil then reduce heat to low, cover, and simmer fro 10 to 12 minutes, or until the sauce thickens.
Add the corn and peas to the meat mixture and spread evenly into an 11 by 7-inch 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 the cheddar cheese on top. Place on a parchment lined half sheet pan on the middle rack of the oven and bake for 25 minutes or just until the potatoes begin to brown. Remove to a cooling rack for at least 15 minutes before serving.