In a large pot, add water and cook on high heat.
Warm up the olive oil in a pan together with the garlic. Fry until slightly brown. Remove the garlic and add the half onion cut into very thin slices.
Cook on low fire until the onion becomes transparent.
Add the tuna and the olives and let it cook for about 10 minutes. As soon as the water boils, add 2 tablespoons of salt and stir to let it dissolve.
Add the pasta and let cook for about 15 minutes (check the cooking time on the bag of your pasta).
When ready, drain it and add it to the sauce. Stir very well and add some cooking water if it is too dry.
Only at the end add some fresh parsley previously chopped and pepper.
Split the fettuccine in two plates and enjoy it warm!