1. Heat your oven to 180°C/Fan 160°C/Gas 4. Put a small, covered pan of salted water on to boil. Put a dry frying pan over a medium heat. Tip in the pine nuts and toast for 2-3 mins, shaking the pan occasionally, till golden. Tip into a bowl and set aside
2. Peel and finely chop the onion. Trim the dry ends off the celery stick and finely chop it to match the onion.
3. The water should be boiling by now so pour in the pearl barley. Cover, bring back to the boil, then turn the heat down and simmer for 20 mins till the barley grains are tender.
4. Put the frying pan back on a medium-low heat for 2 mins, then add the onion and celery with ½ tbsp oil and a pinch of salt and pepper. Fry for 10 mins, stirring now and then, till the veg have softened and look glossy.
5. While the veg fry, slice the top 1cm off each tomato to make a lid, then use a teaspoon to scoop out the seeds (you can discard the seeds or plant them for a tomato patch of your own). Use a small knife to slice out the cores, so you have 2 empty tomato shells. Pop the hollowed out tomatoes on a baking tray. Peel and crush the garlic clove. Finely grate the zest from the lemon. Stir the garlic and lemon into the veg. Stir and fry for 1 min, then take off the heat.
6. When the barley is tender, drain it and stir it into the veg in the frying pan. Tip in the pine nuts and tear in the basil leaves, keeping a couple of leaves back for garnishing. Add the juice from half them lemon. Stir to mix it all together. Taste, and add a pinch more salt or pepper if you think it needs it.
7. Spoon the barley mix into the tomatoes. If you have any stuffing leftover, keep it in the pan with the lid on so it stays warm (not hot). Pop the lids on the tomatoes and then place the baking tray in the oven. Bake for 20 mins, till the onions are tender and juicy looking.
8. While the tomatoes bake, place the salad leaves in a large bowl. Whisk 1 tbsp balsamic vinegar with ½ tbsp olive oil and a pinch of salt and pepper to make a dressing, then add it to the salad leaves. Toss to mix.
9. Serve the baked tomatoes with a pile of the dressed salad leaves and any leftover stuffing heaped up on the side.