Preheat the oven to 375 degrees F (190 degrees F). Grease a 9x13 inch baking dish.
Place the tilapia fillets in the bottom of the baking dish and dot with butter. Season with Old Bay seasoning and garlic salt. Top each one with a slice or two of lemon. Arrange the frozen mixed vegetables around the fish, and season lightly with salt and pepper.
Cover the dish and bake for 25 to 30 minutes in the preheated oven, until vegetables are tender and fish flakes easily with a fork.