Braised chicken with spinach and lentils

Ingredients (serves 4) 1½ tablespoons olive oil 4 x 170g chicken breast fillets, cut into 3cm cubes 1 red onion, finely chopped 1 celery stalk, finely chopped, plus 1 tablespoon chopped celery leaves 2 garlic cloves, finely chopped 2 teaspoons chopped thyme leaves 1 cup (250ml) salt-reduced chicken stock 120g baby spinach leaves 2 x 400g…