01 - Preheat oven to 400°F.
02 - In a large skillet over medium heat, warm the olive oil. Add the finely chopped onion, diced carrots, and diced celery; sauté for approximately 5 minutes until they begin to soften.
03 - Add the minced garlic and cook for 1 minute until fragrant. Introduce the ground lamb; cook until thoroughly browned, breaking up the meat with a spoon as it cooks. Drain any excess fat if necessary.
04 - Stir in the tomato paste, Worcestershire sauce, thyme, rosemary, salt, and pepper. Cook for 1 to 2 minutes, allowing the flavors to meld.
05 - Pour in the beef or lamb broth and bring the mixture to a simmer. Cook uncovered for 10 minutes, stirring occasionally, until the gravy has slightly thickened. Stir in the frozen peas just before removing the skillet from the heat.
06 - While the lamb filling simmers, place the peeled and chunked potatoes in a large pot. Cover them with cold, salted water and bring to a rolling boil. Reduce heat and simmer for 15 to 20 minutes, or until the potatoes are fork-tender.
07 - Drain the cooked potatoes thoroughly and return them to the pot. Add the unsalted butter and warmed whole milk. Mash until the potatoes are smooth and creamy. Season generously with salt and pepper. If desired, stir in the shredded Irish cheddar cheese.
08 - Transfer the prepared lamb mixture into a 2-quart baking dish, spreading it evenly across the bottom. Carefully spread the mashed potatoes over the lamb filling, ensuring it covers completely. Use a fork to create decorative ridges on the potato topping; this will help achieve a crispy crust during baking.
09 - Bake the shepherd's pie for 25 to 30 minutes, or until the potato topping is golden brown and the filling is bubbling around the edges. For an even crispier and more golden finish, broil for 2 to 3 minutes at the very end, watching carefully to prevent burning.
10 - Allow the pie to rest for 10 minutes before serving. This allows the filling to set and prevents it from falling apart when portioned.