Boil up plenty of salted water for the pasta. Grate the parmesan. Peel garlic and chop very finely. Cook the pasta in salt water according to the instructions on the packet.
Mix the pesto with half the parmesan, oil, garlic and almonds with a whisk. Season to taste with salt and pepper. Wash the basil and remove the leaves. Drain the pasta and collect some cooking water.
Put the spaghetti back in the pot. Mix with pesto, basil and 4-5 tablespoons of cooking water. Serve with the remaining Parmesan cheese.