Scrub the oysters and rinse them off. Shuck the oysters(if you have trouble shucking try placing the oysters in a shallow pan of boiling water for a minute first). Always smell a freshly shucked oyster to make sure it's fresh, they should have no strong odor and merely smell like the ocean.
Once the oysters are shucked, remove the meat and rinse it off in cold water, being sure to remove any shell fragments or the beginnings of a pearl(these are murder on your teeth) Discard the flat top half of the shell and keep the lower bowl shaped half, rinsing them out as well. Dry the meat by patting between 2 paper towel sheets.
Whisk the mayonnaise, miso paste and peppers together. Set aside.
Lightly saute the mushrooms and onion in butter to remove some of the water.
Heat oven to 400°F.
Spread a thin layer of mayo mixture on the bottom of each shell.
Cut the oyster meat into 4-6 pieces per oyster and divide evenly onto the shells.
Divide the onion & mushroom mixture evenly into shells and top off thickly with the mayonnaise mixture, being sure to completely cover everything in the shell with the mixture.
Place the shells on a baking sheet or arrange them in a shallow pan as level as you can.
Place oysters in the oven on the top rack and bake for 20-25 minutes. The tops should be a nice dark brown when done.
Remove oysters from the oven, placing a small amount of Masago on each and serve with a lemon wedge.