3/4 pound russet potatoes, peeled and diced
2 tablespoons whole milk
2 tablespoons unsalted butter
1 egg yolk
Salt and pepper to taste
1 tablespoon vegetable oil
1/2 yellow onion, diced
2 cloves garlic, diced
1 medium carrot, diced
1 rib celery, diced
3/4 pound ground lamb
1 tablespoon tomato paste
1/2 cup chicken broth
1 teaspoon Worcestershire sauce
1/2 cup frozen English peas
1 teaspoon rosemary, dried
1 teaspoon thyme, dried
1 tablespoon cornstarch
1/4 cup grated Parmesan cheese
Place the potatoes in a medium pot and cover with cold water. Add a pinch of salt.
Bring to a boil and cook 15 minutes.
Drain and return to the pot along with the milk, butter, egg yolk, salt and pepper.
Mash until everything is homogeneous and smooth. Set aside.
Heat the oil in a large, deep pan over medium heat.
Add the onion and garlic; they should sweat until they begin to become translucent.
Add the carrot and celery, and continue to cook about 3 minutes, until beginning to soften.
Increase the heat to medium-high and add the lamb and a pinch of salt, breaking it up with a spoon and stirring until it has completely browned.
Add the tomato paste, chicken broth, Worcestershire sauce, peas and spices. Stir to combine, and bring the mixture to a simmer.
Reduce the heat to medium and cook for 30 to 45 minutes. Add cornstarch for extra thickness.
Preheat the oven to 350 F.
Pour the lamb mixture into the bottom of a heavy 8-by-8 casserole dish.
Spread the mashed potatoes evenly over the lamb mixture, and top with the grated Parmesan cheese.
Bake for 20 to 25 minutes, or until the top is golden brown and the edges are bubbling.
Allow to cool 5 minutes before serving.