How to Make It
Preheat the oven to 400°. On a large rimmed baking sheet, toss the cauliflower with the oil and season with salt. Roast for 25 to 30 minutes, stirring once, until just tender and lightly browned; let cool.
Meanwhile, in a large saucepan of salted boiling water, cook the fingerlings just until tender, about 8 minutes. Using a slotted spoon, spread the fingerlings on a large baking sheet and sprinkle with 1 tablespoon of the vinegar. Repeat with the baby red potatoes and remaining 1 tablespoon of vinegar; let cool.
In a large bowl, whisk the mayonnaise, shallots, mustard, capers and the 1/4 cup of parsley. Add the cauliflower and potatoes and toss. Season with salt and garnish with parsley. Serve with horseradish.