In a large bowl, toss the onion with the lemon juice and a pinch of salt. Let marinate for at least 10 minutes.
Remove the tough outer layer of the fennel bulbs. Using a mandoline or a sharp knife, thinly slice the bulbs crosswise.
Add the fennel, cabbage, carrot, parsley, and dill to the bowl of marinated onion. Sprinkle with 1 teaspoon salt and lightly massage the mixture with your hands for about a minute to wilt the cabbage a bit. Let the slaw rest for 15 minutes, discarding any liquid that collects in the bowl.
In a small bowl, whisk together the horseradish and vinegar, then whisk in the olive oil. Toss the slaw with the vinaigrette and the mayonnaise and serve.
Amount Per Serving
Calories 206Calories from Fat 153
% Daily Value*
Saturated Fat 2g13%
Trans Fat 1g
Polyunsaturated Fat 4g
Monounsaturated Fat 10g
Vitamin A 2650IU53%
Vitamin C 50mg61%
* Percent Daily Values are based on a 2000 calorie diet.