Preheat oven to 425°F. Line a baking sheet with parchment paper; lightly coat with cooking spray. Cook quinoa in a large skillet over medium-high, stirring occasionally, for 3 to 4 minutes, until toasted and fragrant. Remove from heat; cool 10 minutes. Transfer to a food processor; process for 20 seconds or until finely ground. Stir together flour, paprika, thyme, and 1/2 teaspoon black pepper in a shallow dish. Stir together buttermilk and egg in a second shallow dish. Place ground quinoa in a third shallow dish. Dredge fish pieces in flour mixture; shake off excess. Dip fish in egg mixture; dredge in quinoa, pressing to adhere. Place fish in a single layer on prepared baking sheet. Lightly coat breaded fish pieces with cooking spray. Bake at 425°F for 13 minutes, turning once after 7 minutes, until golden and cooked through. Meanwhile, stir together mayonnaise, yogurt, pickles, vinegar, mustard, dill, and remaining 1/4 teaspoon black pepper in a small bowl. Serve with fish nuggets.