Heat a little oil in a medium oven-proof frying pan on a medium high heat. Add onion and cook, stirring often until onion is soft and golden.

Preheat your oven to 250C (480F). When the onion is cooked, remove from the heat. Stir in spinach, tuna and sour cream. Smooth the top and sprinkle over cheese.

Bake for 10 minutes or until cheese is melted and everything is hot.