Slice and chop the mushrooms into desired size. It's okay to have a small variety of textures. It will add character to the sauce.
In a large pot or casserole dish, melt the butter on low-medium heat. Cook the onions until softened.
At this point, you will begin to add the mushrooms to the pot. Keep in mind, the mushrooms will not all fit in the pot at the same time. They will cook down to a fraction of their original size in only minutes. You will have to gradually add the to the pot. Start with the Cremini Mushrooms, and work your way down the list of mushrooms.
Add the flour, salt, and onion powder. Continue to cook until all the mushrooms fit into the pot and have cooked down.
Add the Brandy. Cook for 2 more minutes.
Stir in the Dill, Parsley, and Heavy Cream. Cook until the sauce thickens.
This is a family-sized portion of the mushroom sauce. I double the recipe for Thanksgiving.