Pour the vegetable stock into a medium-sized saucepan and bring to the boil. Tip the quinoa into the pan, cover and simmer gently for 12–15 minutes. It is ready when the seed begins to come away from the germ. Remove from the heat and drain off any stock that hasn’t been absorbed.
Meanwhile, preheat the oven to 200°C (400°F), Gas Mark 6 and heat the oil in a large frying pan over a medium heat. Add the mushrooms and cook, stirring, for 4–5 minutes, then add the courgettes. Cook for a further 4–5 minutes, then stir in the spring onions, sunflower seeds, basil, lemon rind and cooked quinoa and season well with salt and pepper.
Cut the tops off the tomatoes and scoop out the seeds. Spoon the quinoa mixture into the tomatoes, then top each tomato with the sliced mozzarella. Place on a lightly greased baking tray. Replace the lids, then bake in the oven for 15–18 minutes until the mozzarella melts. Serve with crisp salad leaves.