Preheat oven to 425 degrees. Grease a 9 x 13 inch baking dish.
Sprinkle both sides of tilapia fillets with Cajun seasoning and salt and pepper. Place in prepared baking dish and top with lemon slices. Bake 12-16 minutes, depending on size and thickness of fillets, until tilapia flakes easily with a fork.
Meanwhile, prepare the sauce by combining all ingredients in a small bowl. Serve alongside the fish.