Preheat oven to 400° and spray a 3qt baking dish with a non stick spray. Set aside.
Peel and thinly slice potatoes, approximately 1/8-1/4" thick. Set aside.
In a large saucepan, melt butter over low flame then add onions and garlic. Saute for about 4-5 minutes or until tender.
Sprinkle flour and nutmeg over onion mixture, stir and cook for a couple minutes.
Pour the broth into pan and stir, add salt and pepper, seasoning and heavy cream. Stir until thickened, remove from heat just prior to boiling and add a cup of each cheese, stirring to combine.
Layer 1/3 of the potato slices in a sprayed baking dish then add 1/3 of the sour cream and 1/3 of the cheese sauce over the potatoes.
Top with 1/3 of the Parmesan, 1/3 cheddar jack cheese and 1/3 of sharp cheddar cheese.
Repeat layering 2 more times ending with cheddar cheese and a sprinkle of parmesan. Cover with foil and place in oven. Cook for 1 1/2 - 2 hours, until potatoes are tender.
About 15 - 20 minutes prior to potatoes being done, remove the foil to brown the cheese. Remove from oven, let sit for about 15 minutes to bring the temperature down.
Serve with a sprinkle of fresh parsley if desired.