Add corn to a large saucepan full of water. Once boiling, continue to boil for 5-7 minutes or until corn is bright yellow and tender. Drain liquid and reserve corn.
Slice sausage into 1" rounds and place in a single layer in a skillet. Heat over medium high heat, browning both sides, for about 6-8 minutes total. Remove from pan and set aside.
In the same skillet, add butter, onion, garlic, broth, cayenne pepper, paprika and Old Bay seasoning. Cook over medium high heat until it just begins to boil. Reduce heat to simmer for 6-7 minutes. Remove from heat. Remove about 3/4 cup of sauce and reserve for serving.
Preheat oven to 400 and place an oven bag on a baking sheet. Open the bag and place shrimp, sausage, corn and crab legs.
Spoon in sauce. seal the bag, keeping air around the food, using the tie provided. Gently shake and knead the bag so sauce coats all seafood and corn. Using a knife or scissors, cut 4 slits in the top of the bag to vent it.
Bake for 20-25 minutes. or until shrimp is pink and hot. Remove from oven and let stand for 5 minutes. Cut open bag and serve seafood boil with reserved sauce and slices of lemon to squeeze over the top.