Kosher salt and fresh coarsely ground black pepper
1/3 cup Dijon mustard
1/3 cup mayonnaise
1/2 tsp. chopped fresh rosemary
Heat a gas grill to medium high. Remove the tenderloins from the chicken breasts for more even cooking; reserve for another use. Put the chicken in a large bowl and season with 1-1/2 tsp. salt and lots of fresh coarsely ground pepper. Add the mustard, mayonnaise, and rosemary and mix thoroughly to combine and to coat the chicken well. Spread the chicken out on the grill, watching the heat carefully; there may be a few flare-ups. Cover the grill. Grill the chicken on one side until golden-brown grill marks form, 2 to 3 min. Rotate the chicken 90 degrees and grill for another 2 to 3 min. (to get a crosshatch of golden grill marks); flip and repeat on the other side, grilling for another 4 to 5 min. (for a total cooking time of 8 to 10 min.) until cooked through. The chicken should be firm and golden brown all over. Arrange on a platter to serve family style.