Preheat oven to 180c.
Cut 1.5 cm of the top of each beef steak tomato, and using a large spoon, carefully scoop out the centre core and seeds, then discard.
Rub the inside of each tomato liberally all over with 2tb olive oil, place on an oven baking dish, season with salt and pepper, place to one side.
Meanwhile in a pan heat the remaining oil, add the onion, and cook over gentle heat until they begin to soften.
Add the cumin and coriander, cook together for one minute.
Add the cooked black rice, mix well, then pour over the stock, bring to a simmer, and cook for 12 minutes.