Bring 5 quarts of water to a boil in a 10- to 12-quart pot. Squeeze lemon juice into water, then add the halves. Add onions, garlic, salt, and spice bundle. Reduce heat to a simmer; cover, and cook 10 minutes.
Add potatoes to pot; return liquid to a boil. Reduce heat; simmer, covered, until potatoes are almost tender, 15 to 20 minutes. Add corn; continue cooking 5 minutes more.
Remove from heat. Stir in shrimp, submerging them completely. Cover, and let stand until shrimp are cooked through, about 3 minutes. Ladle 2 cups of broth into a glass measuring cup; drain the rest in a large colander, discarding liquid.
Make spicy butter: In a small bowl, stir together melted butter and hot sauce to taste. Divide butter among small dishes or ramekins.
Divide reserved broth among small bowls for dipping. Transfer shrimp and vegetables to a large platter, and serve with spicy butter, if desired.