Peel the potatoes and cut into 4. Place in a medium pot and cover with cold water. Set over high heat, add a teaspoon of salt, cover and bring to a boil. cook until tender.
While the potatoes are cooking, heat olive oil in a pan over medium heat. Add the onion and sauté 2-3 minutes, add crushed garlic and chopped carrots, sauté until the carrots are tender, about 5-7 minutes.
Add the ground lamb, Cook until no longer pink, about 5 minutes. Season with salt, pepper, chopped rosemary and thyme. Add tomato paste and stir. Add worcestershire sauce.
Add beef broth and frozen peas. Bring to a simmer, reduce heat to low and cook, stirring occasionally, until sauce thickens.
When the potatoes are done cooking transfer them to a bowl. Add butter, milk, egg yolk, salt and pepper. mash the potatoes until smooth.
Preheat oven to 400F (200C).
Layer the meat mixture in a 13"X8.5" (33X22cm) ovenproof dish. Spread the puree over the meat mixture.
Bake 20-25 minutes. or until mashed potato is golden brown.