Preheat your oven to 350°F (175°C).
In a large pot, bring salted water to a boil and cook the elbow macaroni according to package directions until al dente. Drain and set aside.
In a large saucepan, heat olive oil over medium heat. Add diced onion and cook until softened, about 3 minutes. Stir in minced garlic and cook for an additional minute.
Add the cooked shrimp and scallops to the saucepan, and cook for about 2-3 minutes until heated through. Remove seafood and set aside.
In the same saucepan, whisk together the milk and flour over medium heat. Add Dijon mustard and Old Bay seasoning, stirring until the mixture thickens and bubbles.
Gradually stir in cheddar cheese and mozzarella cheese until melted and smooth. Season with salt and pepper to taste.
Combine the cooked macaroni and the seafood mixture with the cheese sauce. Mix well to ensure everything is evenly coated.
Pour the mac and cheese mixture into a greased baking dish. If you prefer a crunchy topping, sprinkle breadcrumbs over the top.
Bake in the preheated oven for 20 minutes, or until golden and bubbly.
Remove from the oven, let it cool slightly, and garnish with fresh parsley before serving.