Scrub potatoes, slice into thin rounds (peel on, opt), place in a bowl of water till ready to use.
Slice onions and mushrooms. Using an oven proof skillet, melt 1/2 stick of butter with 1 tablespoon of olive oil, put onion in skillet over low heat and cook till tender (about 30 min), make sure not to burn. Add mushrooms and stir around to coat with remaining oil & butter. Remove from skillet.
Using the same skillet the onions were cooked in, layer potatoes in bottom sprinkling with salt, pepper, and garlic powder about half way through.
Heat milk and butter in microwave for one minute, pour over potatoes. Place onions and mushrooms, on top and sprinkle panko, parm and 1 tsp Mrs. Dash on top. Drizzle with olive oil. Bake 350° for one hour or till a fork goes easily through potatoes.