- With the rack in the middle position, preheat the oven to 400°F (200°C).
- In a large skillet over medium heat, soften the onions in the butter, stirring frequently, for about 20 minutes or until they begin to caramelize.
- Add the wine and tomato paste. Continue cooking, while stirring, until the wine has completely evaporated. Season with salt and pepper. Transfer to a 12-inch (30 cm) round baking dish.
- Meanwhile, in a large pot of salted boiling water, cook the potatoes until very tender. Drain. With a potato masher, purée with the butter until smooth. Stir in the cream. Season with salt and pepper. Spread over the onions. Top with the cheese.
- Bake for 20 minutes. Finish under the broiler for 3 to 5 minutes or until the cheese is lightly golden brown. Let rest for 10 minutes. When ready to serve, sprinkle with the chives.
This dish can be made the day before. In this case, bake the gratin for about 40 minutes so that it’s piping hot.