cup chicken stock
tablespoons unsalted butter, divided
tablespoons chopped fresh rosemary, plus 4 to 6 sprigs for garnish
cup finely chopped onions
cup grated Parmesan cheese, plus more for the table
cloves garlic, chopped coarsely
Kosher or sea salt and black pepper to taste
In a large saute pan over low heat, melt 4 tablespoons butter and add the onions. The butter and onions will slowly cook, becoming deep golden brown after 7-10 minutes. Add the garlic and continue cooking for 2 minutes. Add the chicken stock and the chopped rosemary. Increase the heat to medium-high and cook until reduced by a third, about 6-8 minutes. Meanwhile, in a large pot, add 3 quarts of water and about 3 tablespoons salt and bring to a full rolling boil. Add the spaghetti, return to a boil and cook for 7-10 minutes, until al dente. Drain in a colander and add the pasta to the sauce in the pan. Add the remaining 2 tablespoons butter and the cheese; mix well until the butter is incorporated. Adjust seasoning with salt and pepper to taste. Serve in a big bowl or on 4 individual plates.