Whisk lemon zest, lemon juice, garlic, buttermilk, mayonnaise, maple syrup, and pepper in a small bowl until mostly smooth and combined. Season buttermilk dressing with salt. Do ahead: Dressing can be made 1 day ahead. Cover and chill.

Prepare a grill for medium indirect heat (for a charcoal grill, bank coals on one side of grill; for a gas grill, leave one or two burners off). Place chicken breasts on a plate. Season all over with salt, then drizzle olive oil over both sides.

Arrange chicken breasts, skin side up, over indirect heat, cover, and grill until cooked through and an instant-read thermometer inserted into the thickest part of a breast registers 160°, 10–12 minutes (bone-in breasts will take closer to 20 minutes).

Turn chicken breasts skin side down and move over direct heat. Grill until skin is golden brown and crisp, about 2 minutes. Transfer chicken breasts to a plate and arrange skin side up. Let rest at least 5 minutes.

Heat vegetable oil in a small saucepan over medium-high. Once oil is shimmering, add nigella seeds and cook until fragrant and oil around seeds starts to sputter, about 1 minute. Add scallions, reduce heat to medium-low, and cook, stirring constantly, until fragrant, about 1 minute. Immediately pour mixture into a medium bowl and let cool 5 minutes.

Whisk lemon juice, maple syrup, cayenne, and remaining 1 tsp. salt into nigella seed tadka.

Toss lettuce and greens in a large bowl to combine. Drizzle reserved buttermilk dressing over; toss to coat.

Slice chicken breasts on a diagonal ½" thick. Arrange over salad and spoon nigella seed tadka over.