How to Make It
Put the cabbage, onion and fennel in three separate bowls and cover with ice water; let stand for 30 minutes. Drain the vegetables and spin dry in a salad spinner. In another bowl, toss the cucumbers with 2 teaspoons of salt and cover with ice water. Let stand for 30 minutes, then drain and pat dry.
In a very large bowl, whisk the crème fraîche with the vinegar until stiff. Add the dill and poppy seeds and season generously with salt. Fold in the cabbage, onion, fennel and cucumber and serve right away.