Peel onions and cucumber. Halve cucumber lengthwise and scrape out seeds with a spoon. Cut onion and cucumber into 1 cm (approximately 1/2 inch) cubes. Transfer onion and cucumber to a bowl and toss vigorously with salt. Let stand about 1 hour. Rinse vegetables thoroughly under cold water and drain well. In a bowl, mix together yogurt, sugar and lemon juice.
Fold in onion and cucumber mixture and season with pepper. Rinse tomatoes and halve crosswise. Remove seeds and thinly slice. Sprinkle Raita with cumin and turmeric and garnish with tomatoes and herbs.