Add the quinoa to a saucepan of boiling water, then simmer for 10–12 minutes until tender. Drain, rinse in cold water and drain again.
Meanwhile, cut the tops off the tomatoes and hollow out the centres with a teaspoon. Put half the tomato pulp and seeds in a bowl, discarding the rest, add the onion, peppers, chilli, parsley, coriander and sunflower seeds and mix well.
Stir the sesame oil and soy sauce together and pour into the bowl. Mix well, then add the quinoa and mix again. Season with pepper; the soy sauce is salty, so you won’t need to add extra salt.
Sit the tomato shells on a baking sheet. Spoon the quinoa mixture into the tomatoes, drizzle with the olive oil and bake a preheated oven, 190°C (375°F), Gas Mark 5, for 30–35 minutes until the tomatoes are tender.