Melt 1/2 cup butter in a heavy skillet over high heat.
Add onions and saute until soft.
Stir in oysters and cook briefly, just to heat through.
Add parsley and lemon juice.
Continue stirring and cooking for about 5 minutes.
Fold in diced bread, stir, and cook until all pan juices are absorbed.
Remove mixture from heat and cool.
When cool, mix in the egg yolks.
Season with salt and pepper to taste.
Preheat oven to 375 degrees.
Cut a pocket 3/4 of the way through the thick part of each fillet.
Gently stuff oyster dressing into pocket.
Place stuffed salmon in a baking dish, drizzle over melted butter, and cover loosely with tinfoil.
Lower oven temperature to 300 degrees.
Bake for approximately 15 minutes, or until salmon is done.