Roast Trout and Vegetables with Horseradish Vinaigrette

INGREDIENTS

• 1 small potatoes (such as red-skinned or fingerling), unpeeled, halved lengthwise
• 1 medium fennel bulb, cut lengthwise into 1/2-inch wedges with some core intact
• 4 small shallots, halved
• 6 olive oil, divided
• 4 Kosher salt, freshly ground pepper
• 8 4-ounce trout fillets
• 2 prepared horseradish
• 2 red wine vinegar
• 1 whole grain mustard
• 1/4 coarsely chopped fresh flat-leaf parsley