Prepare spaghetti in plenty of boiling salted water according to instructions on the packet. Peel onion and garlic and dice finely. Heat the fat in a saucepan. Sauté onion and garlic in it until transparent.
Add cream and bring to the boil. Bind with sauce thickener. Add capers. Season to taste with salt, pepper, chilli and lime juice. Drain tuna and pluck into small pieces. Stir into the sauce.
Drain the spaghetti and let it drain. Mix spaghetti and sauce. Arrange on plates. Garnish with basil and lime pieces.