Put the cabbage and peppers in a spacious bowl. If stringly, peel the celery, then slice it. Chop up any celery leaves as well. Slice the green onions into rounds, and add the celery and onions to the bowl.

In another bowl, combine the lemon zest and juice, sugar, salt, pepper, mustard, horseradish, and celery seeds; stir to dissolve the sugar, then whisk in the oil.

Pour the dressing over the vegetables, add the parsley, and toss well. Cover and chill for 3 hours before serving.

Variation: Add a few tablespoons finely sliced lovage leaves to the coleslaw with the parsley and celery leaves. Just before serving, add 1 cup toasted sunflower seeds and toss well.