Heat olive oil in a pot. Sauté the lentils and diced onions for 1-2 minutes. Add tomato paste and sauté briefly. Deglaze with approx. 400 ml water and cook until soft for approx. 30 minutes.
Approximately 10 minutes before the end of cooking, add diced carrots and potatoes and season to taste with salt, pepper and vinegar. Add spinach 2-3 minutes before cooking. Stir the starch and 2 tablespoons of water until smooth and use to thicken the stew, bring to the boil and serve.