Prepare the spaghetti in boiling salted water according to the instructions on the packet.
Peel onion and garlic and chop finely. Drain the tuna. Squeeze the lemon. Sauté the onion and garlic in a saucepan in olive oil. Add the tuna, fry briefly, deglaze with the lemon juice. If you like, you can now add a dash of white wine and let it boil down briefly. Add the cream. Bring to the boil and cook over low to medium heat for about 3 minutes. Season to taste with salt and black pepper.
If the sauce is too runny for you. can thicken them with a little flour or a sauce thickener.