Position a rack in the center of the oven; preheat to 300 degrees . Spread the quinoa on a parchment-lined baking sheet; bake until lightly toasted, 25 to 30 minutes. Let cool, then transfer to a bowl, breaking up any clumps. Discard the parchment; reserve the baking sheet.
Increase the oven to 425 degrees . In a large bowl, combine the chicken, mustard and thyme; season with pepper. Mix with a rubber spatula to coat. Place a rack on the reserved baking sheet. Dip the chicken in the quinoa, coating well on both sides, then place on the rack.
Spritz the chicken with cooking spray; bake until just cooked through, 15 to 20 minutes. Sprinkle with thyme.