Fill a small pot with water and bring to a boil. In a separate pan melt butter, then add garlic, spinach, and lemon juice. Cook until the spinach is wilted, about 3 minutes, then add cream, salt, rosemary, and parmesan. Simmer on low, stirring occasionally, for about 15 minutes, until the sauce is thick and creamy. In the meantime, cook the pasta in the boiling water according to package directions. When the pasta is cooked and the sauce is thick, drain the pasta and add it to the sauce. Toss to coat, and garnish with extra shredded parmesan if desired.