Add the lentils to a saucepan of boiling water and simmer for 15–20 minutes until just tender. Meanwhile, heat 1 tablespoon of the oil in a large frying pan, add the chicken and onion and fry for 10 minutes, stirring until the chicken is browned.
Stir in the rosemary and paprika and cook for 1 minute, then mix in the tomatoes and seasoning and cook for 3–4 minutes, stirring until the tomatoes are just beginning to soften.
Drain the lentils into a sieve, add the remaining oil, vinegar and seasoning to the base of the dry pan and mix together. Return the lentils to the pan and mix in the spinach. Cook for 1 minute, stirring until the spinach is just beginning to wilt.
Spoon the lentil and spinach mixture on to serving plates and top with the chicken mix. Drizzle with a little extra balsamic vinegar, if liked. Serve immediately.