Start by preparing the infused dairy. Add the 120 ml cream, 60 g crème fraîche, 60 g melted salted butter, 1 bay leaf, 1 garlic clove, and thyme sprigs to a small saucepan. Warm gently over low heat until it begins to steam, then keep it on the lowest heat to infuse while the rest of the recipe comes together.

Thinly slice the 2 brown onions and place them in a wide pan with a generous drizzle of olive oil. Cook over low heat for at least 30 minutes, stirring often, until the onions are deeply caramelised, soft, and jammy. Season with salt and pepper. Add the 2 tablespoons brandy and cook for another minute or two until the alcohol has evaporated and the flavour has mellowed. Set aside.

While the onions cook, peel the 1.5 kg potatoes and place them whole into a pot of cold, generously salted water. Bring to a gentle boil, then reduce to a steady simmer. Cook until a knife slides through easily or they reach an internal temp of 93°C (200°F), about 25 to 35 minutes depending on size.

Drain the potatoes and let them steam in the colander for a couple of minutes. Cut them in half and slip off the skins. Pass the hot potatoes through a ricer back into the pot (or mash gently with a masher, avoiding overworking).

Strain the infused dairy, discarding the herbs and garlic. Fold the warm mixture gradually into the potatoes until smooth and creamy, adding it slowly so the mash stays fluffy and light. Cut about two-thirds of the 200 g Gruyère into small cubes, then fold those into the hot mash, along with 10 g of finely chopped chives. Taste and adjust seasoning.

Dot the caramelised onions over the mash in several spoonfuls rather than one heap. Gently fold two or three times to create visible streaks of onion rather than an even mixture.

Use a large ice-cream scoop to portion the mash into your serving dish. Grate or shred the remaining Gruyère over the top. Place under a hot grill for 5 to 10 minutes until the cheese is bubbling and lightly golden and the mash is heated through. Serve immediately.