Peel and dice the onion. If you're using green onions, save the green parts as a topping.
Heat a large saucepan with vegetable oil. When hot, sauté the onion over medium heat until fragrant.
Add the sesame oil and the rice to the pan. Sauté for 2 minutes.
Add in the frozen carrots, frozen peas, vegetable broth, and tamari. Cook over medium heat until the liquid has almost evaporated.
Make sure the rice is cooked. If not, add a splash of vegetable broth or water and simmer until cooked.
Optionally top with sesame seeds and the green parts of the green onions.
Notes
Rice: to be able to make this recipe in only 20 minutes, you need to choose a quick-cooking rice that has a cooking time of 10-15 minutes. You can use brown rice but you may need to adapt the cooking time accordingly and only add the frozen vegetable when there are 10 minutes of cooking left.
Vegetable broth: I recommend jarred or boxed vegetable broth made with minimal ingredients. You can also use bouillon powder.
Tamari sauce: make sure it's 100% gluten-free if needed.