Peel, halve and cut onion into thin rings. Peel and finely chop the garlic. Blanche the tomatoes in boiling water, then rinse with cold water, peel and cut into wedges.
Heat oil in a large skillet and saute the onion and garlic. Add tomatoes and cook for about 10 minutes. Whisk the eggs and the water and season with salt and pepper. Rinse herbs, shake dry and finely chop. Stir in half the herbs into the eggs and pour the mixture over the vegetables in the pan and let thicken over low heat. Garnish omelet with remaining herbs. Serve immediately.