If your shrimp come with veins, peel the shell, remove the tail, and devein by scoring the back of shrimp and removing the black veins. Put the shrimp in a small bowl and add 1 Tbsp sake. Coat the shrimp well with sake to remove the smell.
Cut the chicken into small bite-size pieces, about 1-inch (2.5 cm) cubes. Put them in a small bowl and add 1 Tbsp sake to remove the smell.
Thinly slice the onion. Cut the mushrooms into ¼-inch (6 mm) thick slices.
Heat a large frying pan over medium heat and add 1 Tbsp olive oil. I use a Le Creuset 3.75QT braiser, which you can put in the oven and serve directly. Add the shrimp in a single layer.
Season with salt and black pepper and cook until the bottom side changes color.
Flip and cook the other side. When both sides achieve nice pink color, transfer to a plate. Do not overcook the shrimp. The remaining heat will continue to cook the shrimp, and later when the shrimp is cooked in the white sauce.
In the same pan, add the 4 Tbsp (56 g) butter to the pan and let it melt over medium heat. Don’t worry, this 4 Tbsp is for making the white sauce; you will need this much fat to make enough white sauce for your macaroni. Add the onion and sauté until it is coated with butter.
Add the chicken and cook, stirring occasionally, until no longer pink. Season with salt and black pepper.
When the chicken is no longer pink, add the mushrooms and coat with the oil.
Add the shrimp back into the pan. Add 1 Tbsp mirin to the mixture and quickly coat with the ingredients.
Turn the heat to medium-low and gradually add 6 Tbsp all-purpose flour to the mixture, stirring to combine with the ingredients.
If the bottom of the pan is getting burnt, turn off the heat until you blend the flour with ingredients, then turn the heat back on to continue this process. Once the flour is well blended, cook for 2 minutes.
Then gradually add the milk, ¼ cup at a time.
Blend with the mixture well before you add the next ¼ cup of milk. The mixture will continue to thicken as you cook.
Once all the milk is incorporated, add heavy whipping cream and cook for 3-5 minutes, stirring frequently.
At this stage, you need to taste the white sauce and add salt and white pepper to taste. Remember to season a bit more than you would normally do. You will add in macaroni later, which makes the sauce less salty. Turn off the heat and set aside.
To cook the pasta, bring the water to a rolling boil. Stir in salt (I added 1 Tbsp salt to 3 QT of water). Then cook the pasta al dente according to the package directions. Drain well.
Stir the white sauce on medium-low heat and add the drained macaroni to combine with the sauce.
To Assemble & Bake
Preheat the oven to 450ºF (230ºC). For a convection oven, reduce cooking temperature by 25ºF (15ºC). If you are using the oven-safe braiser, skip this step. If you are using individual gratin baking dishes, grease them with softened butter. Divide the macaroni mixture into 4 baking dishes.
Divide the mixture – each Staub 6.5” baking dish should be filled 90%.
Grate the Gruyère and Parmigiano Reggiano and generously sprinkle over the macaroni mixture.
Sprinkle panko on top of the cheese, a bit less than 1 Tbsp for each baking dish. Broil high for 3-5 minutes (depending on the distance from the heat source), or until golden brown. If you don't have a broiler, bake at preheated 450ºF (230ºC) oven for 15 minutes, or until golden brown.
When the cheese is melted and panko is nicely charred, remove from the oven.
To Serve and Store
Sprinkle finely chopped chives and serve immediately. You can keep the leftovers in an airtight container and store in the refrigerator for up to 3 days and in the freezer for a month.