Season a salmon fillet with salt generously and let it sit in refrigerator at least 2 hours before cooking, preferably. Tear or slice mushrooms into bite sizes.
Preheat broiler. Use paper towels, pat dry salmon fillet. Place a salmon on a rack with a sheet tray. Position the salmon about 6 inches from the heat source and broil until it flakes for a few minutes or more. It is easy to burn so watch it carefully. When it cools enough to handle, flake the salmon.
Heat a saute pan over medium heat, and add olive oil. Add garlic; cook until softened but not colored. Reduce heat to medium-low; add mushroom and cook until softened. Stir in salmon flakes.
Add the udon noodle to the boiling water; cook until to the package instruction. Drain well in colander.
Add the udon to the pan; toss with soy sauce and butter until butter is melted and everything combined. Season to taste with more salt. Serve with green onion and shichimi togarashi.
Notes
Shimeji mushroom, oyster mushroom, or maitake mushroom are recommended!