Put a pot with salted water over high heat until it boils. Meanwhile, melt the butter in a medium-sized saucepan on medium heat. Add the chopped onion and saffron and saute them, stirring frequently until they are softened, for about 2 minutes. Add the cream, 1/4 teaspoon of salt and pepper (depending on how spicy you want it) and the tomatoes after you have chopped them. Increase the temperature to moderate to high and simmer until it thickens slightly, for about 2 minutes. Add the lemon juice and vanilla and stir well.
Add the pasta to boiling water and cook according to the instructions of the packaging and depending on how you want your pasta. Keep 1/4 cup of pasta water aside. Drain the pasta and transfer them to the pan with the sauce. Add the water you kept in the previous step, Parmesan cheese and the half green onion. Serve with the lemon zest, the rest of the green onion and all the extra Parmesan you like.