Pressure Cooker French Onion Soup Recipe

unsalted butter, plus more for toasts (3 ounces; about 90g)

yellow or mixed onions, sliced 1/8 inch thick (1.4kg; about 4 to 5 large onions) (see note)

baking soda

Kosher salt and freshly ground black pepper

dry sherry, such as Amontillado

(about 2L) homemade or store-bought low-sodium chicken stock

sprigs thyme

bay leaf

Asian fish sauce (optional)

cider vinegar

bowl-size slices rustic bread, toasted until crisp

medium clove garlic

Gruyère cheese, grated

Freshly minced chives, for garnish