1 lb boneless, skinless halibut fillet, cut into 4 pieces
Arrange 1 oven rack on top and 1 rack on bottom third of oven. Preheat oven to 400°F. Line 2 large baking sheets with parchment paper.
In a large bowl, add parsnips and mist with cooking spray. Season with salt and pepper and toss to combine. Arrange parsnips in a single layer on 1 baking sheet. To bowl, add potatoes and mist with cooking spray. Season with salt and pepper and toss. Add potatoes in a single layer to remaining baking sheet. Transfer 1 sheet to top rack and 1 sheet to lower rack and bake for 30 minutes, until golden brown and lightly caramelized, switching racks and using a thin spatula to turn parsnips and potatoes halfway through.
Meanwhile, prepare tartar sauce: In a small bowl, combine yogurt, pickle, mayonnaise, lemon juice, Dijon, salt and pepper and, if desired, hot sauce, until combined. Cover and refrigerate until needed. (MAKE AHEAD: Tartar can be prepared up to 48 hours in advance.)
In a small shallow dish, combine flour, lemon-pepper seasoning, salt and pepper. In a separate shallow dish, whisk egg, 2 tbsp water, salt and pepper. In a third shallow dish, combine quinoa flakes, salt and pepper. Lightly dredge 1 halibut fillet in flour mixture, shaking off excess. Transfer fillet to egg mixture and turn to coat. Transfer fillet to quinoa flake mixture, lightly pressing fillet into flakes to adhere. Transfer fillet to a clean, flat surface and set aside. Repeat with remaining fillets.
Combine parsnips and potatoes on 1 baking sheet and set aside. Arrange halibut on empty parchment-lined sheet and mist tops with cooking spray. Transfer halibut to top rack of oven and bake for 25 minutes. Place tray of parsnips and potatoes on lower rack and bake for 10 more minutes, until halibut is golden brown, crisp and flakes easily when tested with a fork. Remove halibut, parsnips and potatoes from oven. Serve immediately with tartar sauce on the side for dipping.