Sauté the onions in 1-1/2 Tbsp butter over medium heat, stirring to brown evenly to an amber color.

Add chicken broth and bring to a boil. 

In a separate pan, sauté the mushrooms in 1 1/2 Tbsp butter until brown. 

Add Marsala to the mushrooms, bring to a boil.

Combine the onion and mushroom mixture and add parsley. 

Place halibut in a glass casserole dish and cover with onion and mushroom mixture.

Cover and bake at 350 degrees F. for 20 minutes or so depending on the thickness of the fish.