Fill an 8 - 10-quart stock pot with a strainer insert about halfway with water. Season water with old bay seasoning, salt, and pepper to taste.
Bring to a boil over high heat, add the butter, potatoes, garlic, lemon juice from one lemon, and sausage. Reduce heat to medium-high.
Cover and cook for 15 minutes. Stirring occasionally.
Next add the corn, onion, mussels, and shrimp. Cook for another 15 minutes, or until the potatoes are soft.
Optional: While seafood boil is cooking, lightly grill lemons halves until grill marks appear. Set aside.
Remove, strain and pour onto a newspaper line table. Place grilled lemons around seafood boil and squeeze over right before eating.