Bring a large pot of salted water to a boil. Cook pasta according to package directions. Reserve at least 1½ cups pasta water; drain the rest.
In a large pot or skillet, melt 1 tablespoon butter. Cook sausage over medium heat, breaking into crumbles, until browned. Transfer to a paper towel–lined plate.
Melt remaining butter in the same pot. Add onion and cook 4 minutes, stirring often. Add garlic and cook 1 minute.
Reduce heat to medium-low. Add 1 cup pasta water, beef broth, ricotta, pesto, and parmesan. Stir until creamy and slightly thickened. Taste and adjust seasoning with salt, pepper, lemon, red pepper flakes, or extra parmesan.
Add cooked sausage and pasta to the sauce. Stir to coat, adding additional pasta water as needed for desired consistency.
Top with extra parmesan or fresh basil, if desired.
Notes
Reserve at least 1-1 ½ cups pasta water before draining! The starch and salt in the pasta water gives the sauce flavor and a silky texture.
Adjust the consistency by adding more or less pasta water.
Taste and adjust sauce to preference. Add a little lemon juice for more acid and tang.