Heat the oil in a large pan over medium-high heat until shimmering. Add the ground lamb, season with salt and pepper, and fry for 5 minutes, until cooked through.
3. Step
Add the onion, carrots, garlic, thyme, and rosemary, stir, and cook for about 10 minutes, or until the veggies are softened and the meat is well-browned.
4. Step
Add the tomato paste and Worcestershire sauce and stir to incorporate. Cook for 1-2 minutes, stirring constantly, until the liquid evaporates and the meat is aromatic.
5. Step
Pour in the red wine and reduce until almost completely evaporated.
6. Step
Add the chicken stock, bring to a boil, and cook the meat until the stock has evaporated, 5 minutes.
7. Step
Meanwhile, cook the potatoes in a large pot of boiling salted water until tender. Drain, then return to the hot pan over low heat to dry out a bit.
8. Step
Transfer the potatoes to a large bowl and mash until smooth.
9. Step
Add salt, the butter, egg yolks, sour cream, and Parmesan cheese. Fold until well combined.
10. Step
Transfer the potatoes to a piping bag or zip-top bag fitted with a star tip.
11. Step
Spoon the lamb mixture into a large baking dish. Pipe the golden mashed potatoes over the lamb.
12. Step
Bake for 30 minutes, until the potatoes are golden brown.
