Step 1

Preheat oven to 350°F. Bring 3 quarts water to a boil in a large stockpot over high. Add 3 tablespoons kosher salt to boiling water. Add pasta; cook, stirring once, 2 minutes less than package directions for al dente. Remove from heat. Drain pasta; do not rinse. Return pasta to stockpot, and gently toss with 1 tablespoon oil.

Step 2

Microwave milk and half-and-half in a large microwavable bowl on HIGH until warmed, 3 to 4 minutes; set aside. Stir together all cheeses in a large bowl; set aside.

Step 3

Melt butter with remaining 2 tablespoons oil in a large skillet over medium. Add onion; cook, stirring occasionally, until softened, 3 to 4 minutes. Stir in flour; cook, stirring constantly, until combined but not browned, about 1 minute. Add thyme and garlic; cook, stirring constantly, until fragrant, about 30 seconds. Gradually whisk in warm milk mixture until smooth. Stir in seasoned salt, liquid shrimp-and-crab boil (if using), hot sauce, and ¼ teaspoon black pepper. Cook, whisking occasionally, until slightly thickened, 5 to 8 minutes. Add 3 cups cheese mixture; remove from heat, and whisk until smooth, about 1 minute. Stir in drained pasta until fully coated.

Step 4

Place shrimp, lobster, crabmeat, remaining ¼ teaspoon kosher salt, and remaining ¼ teaspoon black pepper in a medium bowl; toss gently to combine. Lightly coat a 13- x 9-inch baking dish with cooking spray; pour in half of pasta mixture. Sprinkle half of seafood mixture in an even layer on pasta; sprinkle with 2 cups cheese mixture. Repeat layers once with remaining pasta mixture, seafood mixture, and cheese mixture. Cover with aluminum foil. Bake in preheated oven 30 minutes. Uncover and bake until cheese is melted, about 10 minutes. Serve hot.