Heat olive oil in a medium saucepan over medium heat. Add onion and cook until softened, about 5 minutes.
Add rice and cumin, and cook for 1 minute, stirring constantly.
Stir in water, diced tomatoes, and salt. Bring to a boil, then reduce heat to low, cover, and simmer for 15-20 minutes, or until rice is cooked through and liquid is absorbed.
Notes
Serve hot as a side dish or use as a base for other Mexican dishes.