Step 1
To a large pot over medium-high heat, add the vegetable broth and season lightly with salt. Add the onions, bring to a boil, and cook until the onions are softened, 2–4 minutes. Add the pasta and continue cooking, stirring occasionally, until al dente, 8–10 minutes.
Step 2
Remove from the heat and, without draining the macaroni, stir in the cheese, sour cream, and butter until the cheese is thoroughly melted and the macaroni is coated in a rich sauce. Season to taste with more salt and black pepper, sprinkle with chives and scallions, and serve hot.