Using ricotta as the base of a "sauce" for pasta is common in parts of Italy.
For this recipe, we start by sautéing thin slices of tender broccolette in olive oil, garlic, and red pepper. Once the broccolette is crisp-tender, we add ricotta and mascarpone, lemon juice and lemon zest to the skillet. The cheeses soften and melt into a luscious coating for the broccolette and spaghetti. You can turn up the heat in this otherwise mild recipe by adding more red pepper to your taste. Walnuts add an optional nutty, crunchy finish.
Bring at least 2 quarts of water to a boil in a large pan over high heat. Add 2 tablespoons of salt and the spaghetti and stir for 1 minute. Cook according to package instructions until the pasta is al dente. Reserve about 1/2 cup of the pasta cooking water, and drain the pasta. Set aside.
Meanwhile, trim off any dried-out ends of the broccolette stalks. Then cut the stalks crosswise into 1/4-inch thick slices, stopping at the florets. Cut the florets in half lengthwise if they’re large, and keep them separate from the stem slices.
Place a large skillet over medium-high heat and add the oil and red pepper flakes. Sauté the stem slices, stirring frequently, for 3 minutes. Add the florets and continue cooking, stirring frequently, until the broccolette is just crisp-tender, 4 to 5 minutes longer. Add 1/4 cup of the pasta cooking water, the garlic, lemon zest and 2 tablespoons of the lemon juice, and cook for 1 minute, stirring constantly. Stir in the ricotta and mascarpone cheeses and cook until the mixture is hot, 1 to 2 minutes.
Using tongs, transfer the spaghetti to the skillet and stir to combine. Continue cooking the mixture, adding a bit more pasta cooking water (if necessary) to create a sauce that coats the pasta. Stir in the basil and walnuts, if using, and season to taste with salt and pepper. Add the remaining 1 tablespoon of lemon juice, if desired. Divide among 4 heated plates and garnish each serving with grated cheese. Serve hot.