Preheat your oven to 350°F (175°C).
Carefully slice the top off the sourdough bread loaf and hollow out the inside, leaving about 1-inch of bread on the bottom and sides. Set the bread bowl aside and chop the removed bread into small pieces.
In a large mixing bowl, combine the chopped shrimp, crab meat, cream cheese, mayonnaise, shredded cheese, green onions, lemon juice, garlic powder, salt, and pepper. Mix until well combined.
Spoon the seafood mixture into the hollowed-out bread bowl, filling it generously.
Place the filled bread bowl on a baking sheet and bake in the preheated oven for 25 minutes, or until the filling is heated through and the bread is slightly toasted.
Remove from the oven and let cool slightly. Garnish with fresh parsley, if desired.
Serve warm, using the chopped bread pieces for dipping or as a side.