Step 1: Preheat your oven to 400°F (200°C). Wash and thoroughly dry the Russet potatoes. Prick them all over with a fork. Bake for 1 hour to 1 hour 15 minutes, or until completely tender when squeezed.
Step 2: While potatoes bake, prepare the seafood. In a medium bowl, combine the drained crawfish tails, chopped shrimp, 1 tablespoon of Cajun seasoning, 1 tablespoon of onion powder, and the liquid crab boil. Toss to coat evenly and set aside.
Step 3: Once potatoes are baked, carefully remove them from the oven. Let them cool slightly until they are comfortable to handle. Slice each potato in half lengthwise.
Step 4: Using a spoon, carefully scoop out the potato flesh into a large mixing bowl, leaving about a ¼-inch shell intact to maintain structure. Mash the scooped-out potato flesh.
Step 5: Add the softened cream cheese, 1 tablespoon of garlic powder, salt, and pepper to taste to the mashed potatoes. Mix until smooth and creamy. Set aside the potato shells.
Step 6: In a large skillet, melt 4 tablespoons of butter over medium heat. Add the minced garlic and sauté for 1 minute until fragrant. Add the seasoned seafood mixture and cook for 3-5 minutes, or until the shrimp turn pink and opaque. Do not overcook. Remove seafood from skillet and set aside.
Step 7: In the same skillet, melt the remaining 4 tablespoons of butter. Whisk in the all-purpose flour and cook for 1-2 minutes, stirring constantly, to create a roux.
Step 8: Gradually whisk in the half and half until the sauce is smooth and thickened. Bring to a gentle simmer, stirring frequently.
Step 9: Reduce heat to low and add the shredded American, mozzarella, Monterey Jack, and Parmesan cheeses. Stir continuously until all the cheeses are melted and the sauce is smooth and creamy. Stir in the remaining 1 tablespoon of Cajun seasoning and 1 tablespoon of onion powder.
Step 10: Gently fold the cooked seafood mixture into the cheesy sauce. Be careful not to break up the seafood too much.
Step 11: Combine the mashed potato mixture with the cheesy seafood sauce. Mix well until everything is evenly distributed.
Step 12: Carefully spoon the cheesy Cajun seafood mixture back into the reserved potato skins. Heap them generously.
Step 13: Place the stuffed potatoes on a baking sheet. Bake in the preheated 400°F (200°C) oven for 15-20 minutes, or until the tops are golden brown and bubbling. If desired, you can briefly broil for 1-2 minutes for extra crispness on top (watch carefully to prevent burning).
Step 14: Garnish with fresh chopped chives or green onions before serving. Serve hot and enjoy!