Boil the lentils in plenty of water so that they still have a bite (they must not disintegrate!).
In the meantime, melt the butter in a saucepan, peel the onion and cut it into wedges and cook for about 8 minutes until translucent. Add the garlic and cook for a minute. Add the vinegar and all the spices and sauté for two minutes. Drain the lentils and add to the spice mixture with the spinach. Heat until the spinach has crumbled slightly. To taste. Serve immediately.