Break the spaghetti into three parts. Boil one and a half liters of water with a slice of onion; when it starts to boil, add the spaghetti, let it boil for 15 minutes, remove from heat, and drain.
In a deep skillet, add the butter, broth, pepper, garlic salt, coarse salt, and basil, allow everything to season well, add the spaghetti, and toss in the mixture until everything is well combined, cover, and let it simmer on low heat for 5 minutes.
When serving, sprinkle with cheese and apple slices.