In a large paella pan or wide skillet, place the olive oil over medium heat.
Add the diced onions, green bell pepper, and garlic. Sauté until the vegetables are soft and translucent, about five minutes.
Stir in the diced tomatoes and cook until they break down and form a sauce, about five minutes.
Pour in the washed Japanese short grain rice and stir to coat it well with the vegetable mixture, cooking for about two to three minutes.