Cut salmon into ½ inch sized cubes. Marinade salmon with shio koji and rest it for 30 minutes.
Preheat oven to 400F.
Place marinaded salmon on parchment sheet lined baking pan and bake for about 7-8 minutes. Cool it down.
Boil plenty of water and cook spaghetti with salt for about 7 minutes or until done. Drain but make sure to keep about 2 tablespoons of pasta cooking water.
Heat up a pan over medium heat with butter and cook garlic slices. Add all the mushrooms and scallions and cook for about 2 minutes.
Add pasta cooking water, soy sauce, and oyster sauce. Add drained spaghetti and cook for additional minute. Mix well. Add salmon at the end and mix more.