How to Make It
Bring pomegranate juice and sugar to a boil in a small saucepan over high, and cook, whisking occasionally, until reduced by half, about 16 minutes. Whisk together 2 tablespoons water and cornstarch in a small bowl; add to pomegranate mixture. Boil, whisking constantly, until thickened, about 1 minute. Remove from heat; whisk in mustard and butter until smooth.
Meanwhile, heat a grill pan over medium-high. Season chicken with 3/4 teaspoon salt and 3/4 teaspoon pepper. Brush 1 tablespoon oil on pan. Cook chicken until browned, about 6 minutes. Turn and cook until a thermometer inserted in thickest portion registers 165°F, about 6 minutes, turning if necessary to prevent burning. Remove from pan; cover to keep warm.
Place quinoa, pistachios, pomegranate arils, chopped mint, remaining 1 tablespoon oil, remaining 1/4 teaspoon salt, and remaining 1/4 teaspoon pepper in a large bowl; stir until combined. Divide quinoa mixture and chicken among 4 plates. Spoon 2 tablespoons pomegranate sauce over each serving; garnish with mint leaves.