Stoemp (pronounced STOOMP) is a hearty Belgian dish of mashed potatoes mixed with finely cut winter vegetables. Our version includes leeks, carrots and cabbage, with a good dose of bay and garlic for fragrance and flavor. A tablespoon of white balsamic vinegar brightens the dish while adding a touch of sweetness, but white wine vinegar worked, too. Make sure to rinse the leeks well; their many layers trap dirt and sand. It's best to rinse them after slicing so you can get between the layers. For convenience, cook the vegetables, mash them and season with the vinegar and salt, then cool and refrigerate up to a day in advance. Reheat in a large pot over medium with ½ cup low-sodium chicken broth, covered and stirring occasionally, until warmed through, about 15 minutes. Transfer to a serving bowl, then follow the instructions to create a well and finish the dish with the butter and chives.