Heat a dash of oil in a saucepan, and add the onion. Cook over a medium heat for a few minutes, until it is fairly soft. Add the next 7 ingredients (black beans through black pepper), and cook for a further 5 minutes, adding a small splash of water if necessary. Next, add the diced tomato, and cook for just a few minutes more, until the tomato has softened, but is still holding its shape.
Add the cooked rice, and mix to combine. Ensure the mixture is piping hot before assembling your burritos (again, you can add a splash of water if needed, to help you heat the mixture without it drying out).
To assemble, lay a large flour tortilla on a board, and heap about a quarter of the rice and bean mixture into the centre. Top with your choice of additional burrito fillings - I used grated cheese, lettuce, and sour cream.
Fold in the two ends of the burrito, then roll or fold to fully contain the fillings (see blog post for photos if needed!). Burritos are best eaten immediately, especially if you've mixed both warm and cold fillings.