Preheat the grill to high heat.
Season both sides of the chicken breasts with salt and black pepper.
Place the chicken on the hot grill and cook about 10–12 minutes, flipping only once, until the internal temperature reaches 165°F (74°C).
While the chicken cooks, combine 1/4 cup light sour cream, 2 1/2 tablespoons light buttermilk, 2 tablespoons diced green onion, 1 teaspoon dill flakes, 1 teaspoon onion flakes, and 1/4 teaspoon garlic powder in a food processor.
Blend on high until smooth and well combined; taste and adjust with salt and pepper as needed.
Serve the dip alongside the grilled chicken or spread it on top, then enjoy.