Heat the oil in a heavy, large saucepan over medium-high heat. Add the onion, red peppers and jalapeño and cook, stirring often, until soft and translucent, about 4 minutes. Add the garlic and cook until fragrant, 1 minute, being careful not to let it burn.
Add the rice and corn and saute 1 minute, stirring constantly to toast the rice and coat with oil. Sprinkle with the cumin and 1 teaspoon each salt and pepper. Pour in the chicken stock, raise the heat to high and bring to a boil. Reduce the heat to low, cover and cook until the rice is tender, 12 to 15 minutes.
Fluff the rice with a fork and carefully fold in the cilantro and diced tomatoes. Season with additional salt and pepper to taste.