Heat a large skillet over medium-high heat. Add olive oil and chopped onions. Cook until onions are soft but not browned, about 3 minutes. Add the rice and cook, stirring, for 2 more minutes until the rice starts to look translucent. Put onion/rice mixture into a microwave-safe casserole dish with a lid.
Put chopped tomatoes with their juice, ground cumin, garlic, and jalapeño (if using) into a blender and blend until smooth. Add enough chicken stock to make 3 cups of liquid and blend.
Add the tomato/chicken stock mixture to the rice, stir, cover with the lid, and microwave on high for about 18 to 20 minutes. Stir 3 times while cooking. Remove from microwave, stir, replace lid and let stand 5 minutes.
If not using the microwave, then add the tomato/chicken stock mixture to the rice in the skillet in 1-cup increments, stirring after each addition of liquid until fully absorbed.