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.

Prepare halibut as you desire -- I like it either poached or dusted in flour then sautéed. 

In a sauce pan, simmer garlic and shallots in the sherry until liquid is almost gone. 

Add the chicken broth, simmer until liquid is almost gone. 

Add heavy cream, simmer until it starts to thicken. 

Add butter.  Add the avocado, tomatoes and cilantro and heat through.

Serve halibut over rice and top with the avocado-cilantro cream sauce.