Preheat the oven to 375?F. Grease a 9 x 13-inch casserole with 1 teaspoon olive oil.
To cook the pasta, combine 6 quarts water, 1 tablespoon olive oil, and 1 teaspoon salt in a large pot over high heat. Bring to a boil, add the pasta, and cook until al dente, about 8 minutes. Remove from the heat, drain, and rinse. Toss with 1 teaspoon olive oil; reserve.
Meanwhile, melt 8 tablespoons butter in a medium saucepan over medium heat. Whisk in the flour and cook for 2 minutes, stirring frequently. Add the milk and whisk to combine well. Bring to a boil, stirring frequently, and cook for 5 minutes, until very thick and the floury taste is gone. Season with 3/4 teaspoon salt, 1/4 teaspoon white pepper, and the nutmeg. Remove from the heat and press plastic wrap onto the surface of the sauce so that a skin will not form; reserve.
Combine the remaining 4 tablespoons butter with the remaining 1 tablespoon olive oil in a medium skillet over medium-high heat. Add the onions and cook until softened, about 6 minutes. Add the garlic and cook for 2 minutes longer. Transfer to a mixing bowl and add the shrimp, crabmeat, lobster, herbs, and 1/2 cup of the sauce. Gently toss to combine. Season with the remaining 1/2 teaspoon salt and 1/4 teaspoon white pepper.
Stuff each of the cannelloni with 1/4 cup of the shellfish filling and place them side by side in the prepared casserole. Sprinkle any remaining filling on top of the cannelloni. Pour the sauce over the cannelloni, sprinkle with the grated Parmesan, and bake until heated through, bubbly, and golden brown, about 30 minutes.
Makes about 8 servings