Wash the potatoes. Cook in boiling water for about 20 minutes. In the meantime, clean and wash the leek and cut it into diagonal rings. Wash, clean and quarter cherry tomatoes. Peel and finely chop the vegetable onion.
Heat 1 tablespoon of oil in a saucepan and sauté the onion in it. Add stock and lentils and simmer for 3-5 minutes. Add the leek the last minute. Season to taste with salt and Sambal Oelek. Set aside.
Drain the potatoes, rinse and peel them. Cut into thin slices. Season the marinade with vinegar. Stir in the remaining oil. Add tomato quarters to the potatoes and pour the marinade over them.
Leave to stand for about 30 minutes. Arrange on a plate and garnish with salad leaves.