Preheat the oven to 160 degrees Celsius/fan 140 degrees Celsius/325 degrees Fahrenheit/Gas 3.

Put the tomatoes cut side up on a baking sheet and sprinkle with the herbs, salt and olive oil. Roast in the preheated oven for one hour. Remove from the oven, let cool, then remove any herb stalks. Transfer the tomatoes to a blender or food processor, together with any juices from the baking sheet. Blend to a purée, adding a little of the wine if the mixture is too thick.

Rub the cut side of the garlic around the inside of the fondue pot. Add the tomato purée and wine and bring to the boil on the stovetop. Reduce the heat to a simmer.

Put the grated cheese and flour into a bowl and toss with a fork. Gradually add to the simmering tomato broth, stirring constantly, letting each addition melt into the broth before adding the next. Season with pepper.

Arrange platters and bowls of the cooked potatoes, cornichons, and pickled onions on the table. Carefully transfer the pot to its tabletop burner. Invite guests to take a share of the potatoes, ladle the hot fondue over the top, then eat with the cornichons and pickled onions.