1. Mix mayo and miso paste, stir until the mixture is very smooth
2. Trim the salmon, four pieces of fillet. Make a deep cut in each of fillet without cutting through them completely.
3. Cut four pieces of foil each large enough to enclose a salmon fillet. Spread the Miso-Mayo sauce in the middle of each foil, and then top the onion and sliced mushroom.
4. Place a portion of salmon on the sauce.
5. Spoon the remaining Miso-Mayo sauce into the cuts in the salmon and top with the cheese.
6. Sprinkle with white sesame seeds and shichimi.
7. Seal the foil closely and bake for 10 mintures.(preheat oven to 350F)
8. Open the foil and bake for a further 5 minutes, until the cheese is lightly browned.
9. Transfer the open foil to plates and serve immediately.