Begin by heating 1 tablespoon of vegetable oil in a large pan and adding 2 chopped onions. Fry them for 7 minutes until softened and slightly browned, then stir in 450 g of ground lamb and cook until crumbly and evenly browned.
Add 2 tablespoons of plain flour, stirring well to coat the meat. Continue building the filling by incorporating 2 finely chopped carrots, 2 tablespoons of tomato puree, 2 tablespoons of Worcestershire sauce, 1/2 teaspoon each of dried thyme and rosemary, and 300 ml of vegetable stock.
Season generously with salt and pepper, then bring the mixture to a boil. Cover the pan and simmer for 30 minutes, stirring occasionally, until the meat becomes tender.
While the meat mixture simmers, prepare the potato topping by peeling and chopping 1 kg of floury potatoes and cooking them in a large pan of boiling salted water until tender. Drain them thoroughly and return them to the pan, then cover and leave to steam.
Dry the potatoes by heating uncovered for 5 minutes. Mash them until smooth, then incorporate 25 g of butter, salt, pepper, and 5 tablespoons of milk until you achieve a smooth consistency. Preheat the oven to 200°C/gas 6.
To assemble the shepherd’s pie, transfer the meat mixture into a 1.7 litre ovenproof dish. Spoon the mashed potatoes over the top and smooth them with a fork to seal in the filling and create an even layer.
Bake for 30 to 35 minutes until the top becomes crisp and golden brown. Allow the pie to cool slightly before serving for the best presentation and easier portioning.