Heat oil in a frypan over medium heat. Add onion, and sauté for 3 minutes. Add tomatoes, zucchini and capsicum. Stir. Season to taste with salt and black pepper. Reduce heat, cover, and simmer for 5 minutes.
Stir in rice and water. Cover, and cook over low heat for 20 minutes.
Use vegetable or chicken stock instead of the water for richer flavour.