Cook pasta in heavily salted water until al dente. Reserve 1 cup pasta water before draining.
While pasta cooks, make pesto: blend parmesan, garlic, olive oil, salt, and a handful of spinach (a stand-in for basil that works beautifully) in a blender or food processor until smooth. Taste and adjust salt and garlic.
Toss hot pasta with pesto immediately. The heat will bloom the flavors.
Add pasta water a splash at a time until the pesto coats every strand evenly and looks glossy, not dry.
Taste. Add more parmesan if needed.
Serve immediately — pesto pasta is best fresh and hot.