Heat 3 tablespoons butter in a large saucepan over medium-high heat. Add 1 finely chopped onion and cook, stirring, until softened, 4 minutes. Stir in 2 cups basmati rice, 2 teaspoons kosher salt and 1/2 teaspoon saffron threads. Add 1 1/2 cups each water and low-sodium chicken broth. Bring to a simmer, reduce the heat to low, cover and cook until the rice is tender and the liquid is absorbed, 15 minutes. Let sit, covered, 5 minutes, then fluff with a fork. Stir in 1/4 cup chopped cilantro and 2 sliced scallions.