In a large stockpot mix water, beer (if using), salt, Old Bay seasoning, thyme sprigs, bay leaves, cayenne or pepper flakes, garlic, parsley, lemon quarters, and onion quarters.
Stir gently and bring mixture to a boil. Add potatoes and corn and cook, uncovered, over medium high heat until potatoes and corn are fork tender about 4-5 minutes (you want potatoes to still be firm, not mushy).
Add the sausage and shrimp to the pot and cook for approximately 1-2 minutes or until shrimp are just barely cooked and lose opacity. (Don't overcook the shrimp or they will be tough.)
Remove the sausage, shrimp, potatoes and corn from the pot with a slotted spoon and place in a large serving bowl. Cover with tinfoil. Discard the broth.
In a large saucepan melt butter over medium heat. Add the garlic and cook for 1 minute until garlic is tender. Stir in lemon juice, Old Bay seasoning, and parsley. Add salt and pepper to taste if needed.
Pour the butter sauce over the shrimp and sausage boil and stir to coat everything. Serve immediately.
Serve with additional Old Bay seasoning, lemon, and melted butter for dipping if desired.
Notes: *If you want to make this more fancy, try adding lobster and crab legs in place of some of the shrimp. *For an authentic seafood boil, serve on top of some white paper or parchment paper. Just dump the drained vegetables and seafood on top of the paper and let everyone dig in. *Any leftovers can be stored, covered, in the refrigerator for 2-3 days. Reheat in the microwave before serving. *Great served with cornbread, french bread, potato salad, mac and cheese, green salad, or baked beans.Calories based on 6 servings. Makes 4-6 servings depending on serving size!