Heat oil in a frying pan, add onions and saute 5 minutes. Add garlic, cook until onions start to brown. Add anchovies. Crush them with a spoon. Add tomatoes, simmer covered to form a sauce about 15 minutes. Drain the tuna and break into large flakes; add tuna to sauce with a little pepper (anchovies and tuna are already salty). Simmer, uncovered, for 5 minutes. Juices will evaporate and sauce will thicken.