Sauce: Heat oil in a small saucepan over medium heat; sauté the garlic and onion for 1 to 2 minutes. Add cream and bring to a simmer. Simmer for about 3 to 4 minutes to reduce slightly. Stir in cilantro, lime juice, and salt and pepper to taste. Simmer for about 1 minute longer. Set aside or refrigerate until nearly serving time.
Fish: Line a baking pan with foil; spray with nonstick cooking spray. Brush fillets lightly with butter; arrange in the baking pan. Sprinkle lightly with salt and pepper. Bake the fish for 20 to 25 minutes, or until the tilapia is cooked through.
Warm the sauce up and drizzle a little over each serving of fish.