Trim beans & cut each into 3 pieces, wash & drain. Put onion & tomato into a heavy pan. Sprinkle with salt & rub in lightly with fingers. Stir in beans, olive oil & sugar. Mix well. Cover & cook for 15 minutes over medium heat. Stir occasionally. The beans will lose their color & soften. When all the moisture is absorbed, stir fry for 2-3 minutes. Add 1/2 cup water, cover & simmer for 40-50 minutes until beans are tender. Add more water if necessary, 1/2 cup at a time. When cooked, the dish should be very moist but not watery. Serve cold.