Spanish tomato and onion base sauce (Sofrito)

1. Halve the tomatoes and grate the cut side on the coarse side of a grater until only the skin is left in your hand; discard the skins.

2. Cut the onions and seeded pepper into very small dice and finely chop the garlic.

3. In a wide, heavy-based frying pan cook the onions and pepper in the olive oil over low heat for 15-20 minutes until completely soft, sweet and lightly caramelised.

4. Add the garlic and fry for 1 minute, then pour in the grated tomato, sugar and salt.

5. Cook over low heat, stirring often, for 30-40 minutes until the tomato liquid has evaporated and the sauce is dark red, thick and jammy, and the oil starts to separate at the edges.

6. Use immediately for rice dishes, stews or fish, or cool, cover with oil and keep in the fridge for up to 1 week.

Tip: Sofrito is the foundation of Spanish cooking – it's worth making a double batch and freezing it in small portions.