Preheat the broiler.
Spray a heavy baking sheet or glass baking dish with vegetable oil cooking spray.Arrange the sole fillets in a single layer on the baking sheet. Season with the salt and pepper and drizzle with the olive oil. Broil until cooked through and the flesh flakes easily with a fork, 5 to 6 minutes. Set aside to cool slightly.
Meanwhile, in a small bowl, mix together the yogurt, agave, and mustard until smooth. Whisk in the lemon juice, chives, salt, and pepper.
Transfer the sole to a serving platter and drizzle with the sauce.