In a small saucepan, stir the fish sauce and sugar over high heat until the sugar dissolves, about 1 minute. Remove the pan from the heat and stir in the cilantro, chiles, garlic, lime juice and crushed red pepper.

Heat a large skillet until hot. Season the sole with salt and black pepper. Add 2 tablespoons of the olive oil to the skillet and swirl to coat the bottom. Add 2 of the fillets and cook over high heat until lightly browned outside and just white throughout, 1 to 2 minutes per side; transfer to plates. Wipe out the skillet and repeat with the remaining olive oil and sole. Pour the lime sauce on the sole and serve.