Aubergines au Gratin

• 4 aubergines, peeled and diced
• salt
• 4 tablespoons olive oil
• butter

Peel and dice aubergines; salt them liberally and leave to drain in a colander for 1 hour. Rinse off salt with cold water and shake diced aubergines dry in a cloth. Combine olive oil and 2 tablespoons butter in a frying pan, and sauté aubergines until golden. Remove from pan, and in the same oil fry onion until soft and just turning golden, adding more oil if necessary. Add tomato concentrate,