1. Preheat oven to 425°F.
2. Place cut potatoes in pot filled with cold water. Add 1 tablespoon of salt. Turn heat to high and cover slightly. Once boiling, uncover and drop heat to medium-high. Cook until potatoes are tender and you can pierce easily with a fork, about 10-15 minutes.
3. While the potatoes cook, make the meat filling. Heat the oil in a medium sauté pan over medium-high heat. Add the onions and cook until soft, about 5-6 minutes. Add garlic and 2 tablespoons of chopped rosemary and stir.
4. Immediately add in the ground lamb, 1 teaspoon of salt and the pepper. Stir occasionally until meat is cooked through, about 8-9 minutes. Drain away any excess fat.
5. Sprinkle flour over the meat. Stir until incorporated and cook for 1-2 minutes. Add in beef broth and Worcestershire sauce.
6. Bring mixture to a boil, then reduce heat to a simmer, stirring occasionally until very thick, 2-3 minutes. Stir in remaining 1 tablespoon of rosemary.
7. Add defrosted peas. Taste and season with additional salt and pepper, if needed.
8. Strain potatoes and place back into their pot. Cook over medium heat until the potatoes are dry, about 1-2 minutes. Take off heat.
9. Add butter and mash. Once mashed stir in heated milk. Stir in lemon zest. Taste and season with additional salt to taste.
10. In 11 x 7 casserole dish, add meat mixture and spread evenly. Top with mashed potatoes starting at the corners working around the edge. This should seal in the meat for better chance of avoiding boil-overs.
11. Use a fork and create fast swirly motions to fluff up potatoes. Bake until potatoes are slightly browned, around 25 minutes.

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