Prepare spaghetti in boiling salted water according to package directions.
Peel and finely dice onion and garlic.
Drain tuna. Squeeze lemon. Sauté onion and garlic in olive oil in a saucepan. Add tuna, sauté briefly, deglaze with 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 a 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 liquid, you can thicken it with a little flour or sauce thickener.
Drain the pasta, mix with the sauce and serve.