In a medium sized saucepan, add the rice, Cajun seasoning, salt and stir.
Add the tomato puree and mix until combined, then add the stock and stir and cover with a lid.
Gently bring to a boil and then move to the smallest ring, turn down to the lowest heat and boil for around 15mins, until the rice is cooked. Checking occasionally to ensure the rice isn't boiling dry.
Whilst the rice is cooking, in a small frying pan, heat the oil and gently fry the onion, peppers and garlic. You can add a couple of splashes of water, this will help the onions not to burn.
When the rice is nearly cooked, add the fried onions and peppers and stir together.
Serve immediately or leave to cool for lunch the next day.