In a large saucepan, heat half the olive oil over a medium-high heat and fry the onion and garlic for 4-5 minutes, stirring occasionally, until they start to soften.
Add the tomatoes and a little seasoning and simmer for 5 minutes.
Meanwhile, pre-heat the oven to 180°C.
Pour the tomato sauce into the base of a baking dish and arrange the sardines on top. Drizzle over the remaining olive oil and season the fish.
Bake for 10-12 minutes until the skin slides easily away from the flesh.
Transfer the tomato sauce onto serving dishes and arrange a sardine on top. Serve immediately.