Prepare a medium-hot grill fire. In a small bowl, whisk the mustard and coriander seeds. Whisk in 3 Tbs. of the olive oil. If the chicken breasts have the tenders attached, remove them and save for another use. Trim any excess fat from the breasts and then rinse and pat dry. Season the breasts with salt and pepper and rub them all over with the mustard-oil mixture. When the grill is ready, grill the chicken until one side is nicely browned and grill marks appear, 2 to 3 min. (There may be some flare-ups at first; if they don’t go out, move the chicken off to the side until they do.) With tongs, rotate the breasts 90 degrees (to get a crosshatch of grill marks) and continue grilling until grill marks form and the sides of the breasts are fully opaque, another 2 to 3 min. Flip the breasts and grill in the same way until the second side is browned and the inside has just a trace of pink, another 4 to 6 min. Transfer to a clean cutting board, cover loosely with foil, and let rest for about 5 min. Meanwhile, whisk the remaining 3 Tbs. oil with the lemon juice, basil, 1/4 tsp. salt, and a few grinds of pepper. Slice the chicken on an angle and serve drizzled with the vinaigrette.