Step 1
Bring a large pot of lightly salted water to a boil. Cook elbow macaroni and 1/2 cup dried onion in the boiling water stirring occasionally until macaroni is tender yet firm to the bite about 8 minutes. Drain.
Step 2
Place lobster and shrimp in a colander. Pour the cooked pasta and onions over shellfish; let drain.
Step 3
Preheat the oven to 375 degrees F (190 degrees C).
Step 4
Warm evaporated milk and cream slowly in a saucepan over medium-low heat. Whisk in flour. Whisk eggs together in a small bowl.
Step 5
Cook sauce until small bubbles form. Add sauce very slowly 1/4 cup at a time to the bowl with the eggs being careful not to cook and scramble the eggs. Slowly mix the egg-cream mixture back into the pan. Continue cooking on medium-low heat stirring occasionally until thick and smooth 5 to 10 minutes.
Step 6
Melt 1 stick of butter in the pot used to cook the pasta over medium heat. Add flour; cook until roux thickens and turns caramel in color about 5 minutes. Stir in Cheddar cheese Swiss cheese and Monterey Jack cheese. Cook stirring constantly until mostly melted about 4 minutes. Pour in the cream sauce and stir until smooth.
Step 7
Stir pasta lobster and shrimp into the cheese and cream sauce to coat completely. Pour mac and cheese into a 9x11-inch baking dish.
Step 8
Melt another stick of butter in the saucepan used to make the sauce. Add onion seasoning and herbes de Provence; stir. Add crackers. Cook and stir over medium heat until coating mixture is lightly toasted about 5 minutes.
Step 9
Cover mac and cheese with thin pats of butter. Top with the cracker coating.
Step 9
Bake in the preheated oven until top is crispy and cheeses are melted about 1 hour.