Warm half the oil in a frying pan over a medium heat. Add the beef, breaking apart the lumps with a wooden spoon; cook and stir until browned and no longer pink then pour away any excess fat and season with salt, pepper and Cajun seasoning.
In a second frying pan, warm the remaining oil over a medium heat and add the onions and garlic; cook and stir until softened.
Rinse the rice under the cold tap. Then pour in two mugs of water and bring to the boil. Crumble in the stock cube. Lower the heat to medium and simmer until the water is absorbed and the rice is tender and fluffy, about 10 minutes. Combine the rice with the beef and stir in the mixed vegetables and onions.
Chop the tops off the red peppers and remove the seeds. Cut in half lengthways then fill each half with filling. Place into an ovenproof baking dish.
Bake in the preheated oven until the peppers are tender, 20 to 30 minutes. Remove from the oven and serve piping hot.