Peel the cucumbers and cut into halves, lengthwise. Scrape out the seeds and cut the cucumbers into 1-inch crosswise slices. Set aside. Heat the oil in a saucepan and sauté the onion until tender, but not browned. Add the tomatoes, salt and pepper to taste, and sugar. Add the cucumbers, stir to mix, cover and simmer very gently for 45 minutes.