Bake the potatoes

• Heat the oven and place the potatoes on a baking tray.
• Bake until completely tender, then let them cool slightly.

Caramelize the onions

• Melt the butter with the olive oil in a frying pan over medium-low heat.
• Add the onions and a pinch of salt and cook slowly, stirring often, until deep golden and jammy.
• Pour in the beef broth and Worcestershire sauce and simmer until the liquid has almost evaporated.
• Stir in the thyme leaves and season with pepper.

Make the filling

• Halve the potatoes lengthwise and scoop the flesh into a bowl, leaving a thin shell.
• Mash the flesh with the sour cream, most of the Gruyère, the Parmesan and the caramelized onions.
• Season with salt and pepper and mix until smooth.

Fill and bake again

• Spoon the filling back into the potato shells.
• Top with the remaining Gruyère.
• Bake until the cheese is melted and bubbling and the tops are golden.

Serve

• Scatter with extra thyme and serve hot.