Prep your ingredients. Mince 2–3 garlic cloves.
Grate 1 cup of parmesan. Zest and halve a lemon. Measure 8 ounces of dry pasta, 2 cups of broth, 1 cup of heavy cream, and 1/3–1/2 cup of pesto.
Warm the pot. In a wide pot or deep skillet, heat 1 tablespoon olive oil over medium.
Add the garlic and a pinch of red pepper flakes. Cook 30–45 seconds until fragrant, not browned.
Add liquids and pasta. Pour in the broth and cream. Stir, then add the pasta.
If using long pasta, gently bend it as it softens so it fits in the pot.
Simmer and stir. Bring to a gentle boil, then reduce to a lively simmer. Cook uncovered, stirring often to prevent sticking, for 8–10 minutes. The pasta should be nearly al dente and the liquid reduced and slightly thickened.
Stir in pesto. Add 1/3–1/2 cup pesto and 1 tablespoon butter (if using).
Stir until the sauce turns a creamy green and coats the pasta. If it’s too thick, splash in a bit more broth or water. If too thin, simmer another minute.
Add cheese and lemon. Turn heat to low.
Stir in 3/4 cup parmesan until melted and smooth. Add 1–2 teaspoons lemon zest and a squeeze of lemon juice to brighten.
Season to taste. Taste and adjust salt, pepper, and lemon. If you like heat, add a pinch more red pepper flakes.
Serve. Twirl into bowls and finish with the remaining parmesan and torn basil.
A drizzle of olive oil on top is a nice touch.