Salad:
Pile white fish, eggs, avocados, apple, greens, and matzo on a large cutting board and chop them all together. (YES, you should have bought the large cutting board!) This way the flavors really meld.
Dressing:
Shake buttermilk, sour cream, scallions, garlic, vinegar, horseradish, mayonnaise, salt, and pepper in a jar.
Store leftover dressing in refrigerator for up to 5 days.
Assembly:
This salad looks best on a platter so you can see all the colors. Skip the bowl! Pile diced beets on the bottom of the platter. Put all chopped salad on top of beets.
Drizzle buttermilk dressing and scatter pieces of matzo on top of salad.
Pass extra dressing when serving.