Rinse the lentils and brown rice and put them in a pot with plenty of water. Bring to a boil over high heat, then simmer over medium-low heat until both are tender, about 25 minutes. Drain well.
Preheat the oven to 200°C (400°F) with a rack in the centre.
Heat the oil in a skillet over medium heat and cook the diced onion until soft, about 5 minutes. Stir in the smoked paprika, cumin and Italian seasoning for 1 minute.
Stir in half the diced tomatoes, the lentil and rice mixture and the salt.
Halve the peppers lengthwise, remove the seeds and set them cut side up in a baking dish. Fill with the mixture and spoon the rest of the tomatoes around them.
Sprinkle with the nutritional yeast, cover with foil and bake for 20 minutes, then uncover and bake 10 minutes more until the peppers are tender.
Serve two pepper halves per person with the tomatoes from the dish.