1. Preheat your oven to 375°F (190°C).
2. Prepare your seafood by cleaning and deveining shrimp, cracking crab legs, and cutting fish fillets into portions.
3. Chop potatoes into small chunks, slice onions, cut corn into smaller sections, and mince garlic.
4. Melt butter and mix it with lemon juice, garlic, Old Bay seasoning, and Cajun spices.
5. In a large bowl, toss seafood, potatoes, corn, and onions with the butter mixture to coat evenly.
6. Arrange all ingredients in a large baking dish or disposable aluminum tray for easy cleanup.
7. Optionally, add sliced Andouille sausage for extra flavor.
8. Cover the tray with foil to keep moisture in while baking.
9. Place the tray in the preheated oven and bake for 25-40 minutes, depending on seafood and vegetable sizes.
10. Check for doneness by ensuring seafood is opaque and potatoes are fork-tender.
11. Remove foil during the last 5 minutes of baking for slight browning.
12. Serve hot with extra melted butter or garlic butter sauce.
13. Garnish with fresh herbs and lemon wedges for added flavor.
14. Enjoy your seafood bake as a hands-on meal, optionally served on newspaper for a rustic experience.