How to Make It
Wrap the thyme, bay leaf, peppercorns and crushed pepper in a piece of cheesecloth and secure with kitchen string. In a medium saucepan, melt the butter. Add the onions, garlic, potato, wine and the herb bundle and bring to a simmer over moderately high heat. Reduce the heat to low, cover and cook until the onions are very soft, about 2 1/2 hours.
Drain the vegetables, reserving the cooking liquid; discard the herb bundle. Transfer the vegetables to a food processor and puree with 2 tablespoons of the cooking liquid and the vinegar. Pass the puree through a sieve, then transfer to a bowl, season with salt and ground black pepper and serve warm.