Nonstick vegetable oil spray
4 teaspoons mustard seeds, divided
1 9-ounce package frozen French-cut green beans, thawed, patted very dry
2 cups very thinly sliced green cabbage
1 small sweet onion (such as Maui or Walla Walla), very thinly sliced
1 tablespoon chopped fresh tarragon
2 tablespoons olive oil
2 tablespoons white balsamic vinegar
3 tablespoons Dijon mustard, divided
1 large egg white
4 boneless chicken breast halves with skin
Coat grill rack heavily with nonstick spray. Prepare barbecue (medium-high heat). Toast 1 teaspoon mustard seeds in skillet over high heat until beginning to pop, 3 to 4 minutes. Place seeds in large bowl. Add green beans, cabbage, onion, and tarragon. Whisk oil, vinegar, and 2 teaspoons Dijon in bowl. Mix into slaw; season with salt and pepper.
Whisk egg white and 2 tablespoons plus 1 teaspoon mustard in small bowl. Sprinkle skin side of chicken with salt and pepper. Brush thickly with mustard glaze; sprinkle with some of remaining mustard seeds.
Place chicken, skin side down, on grill. Sprinkle chicken with salt and pepper; brush with glaze and sprinkle with remaining mustard seeds. Grill 5 minutes. Turn chicken. Grill until cooked through, about 5 minutes. Top with slaw and serve