Hachis parmentier

03 — Method

1. Peel the potatoes, cut into large 4 cm (1½ in) chunks and put them in a large pot of cold salted water. Bring to a boil and cook over medium heat for 20 minutes, until a knife slides in easily.
2. Meanwhile, peel and thinly slice the 2 yellow onions. Remove the germ from the 2 garlic cloves and chop them. Heat 1 tbsp of olive oil in a large skillet over medium heat.
3. Soften the onions and garlic for 5 minutes until translucent but not browned. Add the ground beef and cook over high heat for 8 minutes, breaking it up with a spatula, until well browned.
4. Season the meat with salt and pepper and set aside off the heat. Drain the potatoes and put them through a ricer back into the pot. Stir in the 50 g (2 oz) of butter, then gradually 200 ml (¾ cup) of hot milk, for a smooth, creamy mash.
5. Add the nutmeg and adjust the seasoning. Preheat the oven to 200 °C (400 °F) fan. Butter the 25 × 20 cm (10 × 8 in) gratin dish and press the meat into the bottom in an even layer.
6. Spread the mash evenly on top with a fork, leaving ridges that brown nicely. Sprinkle with the 50 g (2 oz) of grated Gruyère.
7. Bake for 25 minutes at 200 °C (400 °F) fan until golden and bubbling. Let rest for 5 minutes, then serve with a green salad and mustard vinaigrette.