In a large pot, bring lightly salted water to a boil. Add elbow macaroni and 1/2 cup dried onion to the boiling water and stir occasionally until the macaroni is tender but still has a firm texture, which should take approximately 8 minutes. Once done, drain the macaroni and onion mixture.
Put the lobster and shrimp in a colander. Then, pour the cooked pasta and onions mixture over the shellfish to drain.
Preheat the oven to 375°F (190 degrees C).
Take a medium sized bowl and mix the eggs in it then put it aside. In a separate saucepan, warm the evaporated milk and cream over medium-low heat. Incorporate the flour into the mixture and continue to cook until small bubbles begin to appear on the surface. Gradually pour 1/4 cup of the sauce at a time into the bowl with the eggs, being careful not to scramble the eggs. Once blended, gradually incorporate the egg-cream mixture into the saucepan by stirring it in slowly. Cook the mixture over medium-low heat, occasionally stirring, until it reaches a smooth and thick consistency, which typically takes between 5-10 minutes.
In the pot used to cook the pasta, melt 1 stick of butter over medium heat. Once melted, add the flour and continue to cook until the roux thickens and turns a caramel color, which should take around 5 minutes. Add the Cheddar cheese, Swiss cheese, and Monterey Jack cheese to the pot and continue to stir until they are mostly melted, approximately 4 minutes. Finally, pour in the cream sauce and mix until the mixture is smooth.
Incorporate the pasta, lobster, and shrimp into the cheese and cream sauce, ensuring that each piece is evenly coated with the sauce. Then, transfer the mixture into a 9x11-inch baking dish
Melt another stick of butter in the same saucepan used to make the sauce. Add onion seasoning and herbes de Provence and mix. Next, add crackers to the pan and cook over medium heat, stirring continuously, until the coating mixture is lightly browned, which should take approximately 5 minutes.
Spread thin pats of butter over the macaroni mixture. Then, sprinkle the cracker coating on top.
Place the baking dish in the preheated oven and bake until the top is crispy and the cheeses are melted, which should take approximately 1 hour