Spring Onion and Potato Gratin
- 4 Tbs. (1/2 stick) (2 oz./60 g) unsalted butter, plus more for greasing
- 1 yellow onion, thinly sliced
- 3 spring onions or 6 green onions, white and light green portions, thinly sliced
- 2 garlic cloves, minced
- 1/2 cup (4 fl. oz./125 ml) white wine
- 2 fresh tarragon sprigs, plus tarragon leaves for garnish
- 2 cups (16 fl. oz./500 ml) milk
- 1/2 cup (4 fl. oz./125 ml) chicken stock
- 1/2 cup (4 oz./125 g) crème fraîche
- Kosher salt and freshly ground pepper
- 6 Yukon Gold potatoes, sliced about 1/8 inch (3 mm) thick
Preheat an oven to 350°F (180°C). Butter a medium gratin dish.
In a large saucepan over medium heat, melt the 4 Tbs. (1/2 stick) (2 oz./60 g) butter. Add the yellow onion and spring onions and cook, stirring occasionally, until translucent, about 6 minutes. Add the garlic and cook, stirring occasionally, until fragrant, about 1 minute. Add the wine and simmer until reduced by half, about 3 minutes. Add the tarragon sprigs, milk and stock and simmer until the sauce is thick enough to coat the back of a spoon, about 10 minutes. Stir in the crème fraîche. Remove and discard the tarragon sprigs, and season to taste with salt and pepper.
Remove the pan from the heat. Add the potatoes to the pan and stir to coat with the sauce. Transfer the potatoes and sauce to the prepared gratin dish and spread in an even layer.
Bake until the potatoes are tender and golden brown on top, about 1 hour. Let stand for 10 minutes, then garnish the gratin with tarragon leaves and serve. Serves 6 to 8.