Spring Onion and Potato Gratin

Name: Spring Onion and Potato Gratin
Ingredients: 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