Finely chop the onion and garlic. Halve the mushrooms if they are not already bite-sized.
Heat the olive oil in a deep frying pan or wide saucepan over low heat. Gently sauté the onion until soft and translucent.
Add the mushrooms, garlic, and Italian mixed herbs. Stir for a couple of minutes until fragrant.
Pour in the balsamic vinegar and cook for another minute, then stir in the tomato purée followed by the tinned cherry tomatoes.
Season with salt and pepper, cover, and simmer gently for 10 minutes.
Add the drained lentils and chopped kale or cavolo nero. Stir well and cook for a further 10 minutes.
Add the sun-blushed tomatoes and simmer for another 5 minutes.
Stir in the black olives and fresh herbs, then remove from the heat.
Spoon the stew into bowls and top each serving with mini mozzarella balls. Finish with a generous grind of black pepper.