Peel and chop potatoes, place in salted water and boil until tender (15–20 minutes).
While potatoes cook, heat oil in a large skillet over medium-high heat.
Add onion, carrot and celery; sauté 6–8 minutes until softened.
Stir in garlic and tomato paste; cook 1 minute until fragrant.
Add minced lamb; break up and brown thoroughly, about 6–8 minutes.
Pour in wine (if using) and cook until reduced by half.
Add stock, Worcestershire sauce, thyme and rosemary; simmer 10 minutes to reduce and thicken.
Stir in frozen peas, adjust seasoning with salt and pepper, then remove from heat.
Mash drained potatoes with butter and milk until smooth; stir in cheddar and egg yolk if using.
Spread meat filling in a baking dish, top evenly with mashed potatoes and create peaks with a spoon.
Bake at 200°C (400°F) for 20–25 minutes until top is golden and filling bubbles; let rest 5 minutes before serving.