Caramelized Onions by Serious Eats

– 6 tablespoons unsalted butter (3 ounces; about 90 g)
– 3 pounds yellow or mixed onions, sliced 1/8 inch thick (1.4 kg; about 4 to 5 large onions)
– 1/2 teaspoon (2 g) baking soda
– Kosher salt and freshly ground black pepper