Preheat the slow cooker if necessary; see the manufacturer's instructions. Put the lentils into a sieve, rinse well with cold water, drain and add to the slow cooker pot along with the onion, red pepper and tomatoes.
Mix the boiling stock with the tomato purée, thyme and a generous amount of salt and pepper. Pour over the lentils, cover with the lid and cook on high for 3–4 hours or until the lentils are tender.
When almost ready to serve, fork the oil and balsamic vinegar together in a bowl, add the salad leaves and toss gently. Spoon the hot lentils into shallow bowls, draining off any excess cooking liquid. Pile the salad on top and sprinkle with the feta, mint leaves and a little extra pepper. Serve immediately with warm pitta breads. Alternatively, lift the pot out of the housing using oven gloves, leave the lentils to go cold, then make the dressing and toss together with the leaves, feta and mint.