Caramelize the onions

• Heat the olive oil and part of the butter in a frying pan over medium-low heat.
• Add the sliced onions and part of the salt and cook slowly, stirring often, until deeply golden and jammy.
• Add the minced garlic and cook until fragrant.

Brown the lamb

• Push the onions to the side and add the ground lamb, breaking it up with a spoon.
• Cook until browned and no longer pink.

Build the filling

• Add the chopped celery and cook until softened.
• Pour in the white wine and let it reduce.
• Stir in the flour, dried thyme, black pepper and dried parsley flakes and cook briefly.

Simmer the filling

• Add the Worcestershire sauce and beef broth.
• Simmer until the mixture thickens into a rich gravy, then adjust the salt.

Boil the potatoes

• Boil the potato chunks in salted water until fork tender.
• Drain well and let the steam escape.

Make the mash

• Mash the potatoes with the remaining butter and the heavy cream.
• Stir in the grated Gruyère and Parmesan and season with the remaining salt.

Assemble the pie

• Spread the lamb filling in an even layer in a baking dish.
• Top with the cheesy mash and spread it smooth, roughing up the surface with a fork.

Bake

• Bake at 200 °C until the top is golden and the filling bubbles at the edges.
• Let rest for a few minutes before serving.

Finish and serve

• Scatter the chopped fresh parsley over the top.
• Serve hot.