Sea salt to taste
1 medium (1 pound) trimmed and peeled celeriac, cut into 3/8-inch matchsticks
2 medium sweet red peppers, sliced into 3/8-inch strips
1 cup water
½ cup apple cider vinegar
¼ cup white vinegar
2½ tablespoons dry mustard
2½ tablespoons olive oil
Sea salt and freshly ground pepper
1. Bring a pot of salted water to a boil. Drop in the celery root and return to a boil, uncovered. Taste after about 3 minutes – the pieces should be tender on the outside and distinctly crisp on the inside. When ready, immediately lift out with a strainer and turn into a mixing bowl.
2. Drop the peppers into the boiling water and cook until just slightly softened, 3-4 minutes, altogether, then lift out and add to the celery root. Drain off any water in the bowl.
3. To make the dressing, in a nonreactive saucepan, combine the water, cider vinegar, white vinegar, dry mustard, and oil. Bring to a boil over medium-high heat, stirring. Pour over the celeriac and peppers, and stir to blend. Add salt and pepper and taste for seasoning, then cover the bowl and keep it in a cool place until the next day.
4. After sitting for a day, either serve it or turn the mixture into a jar, cover, and refrigerate for use at a later date.