Set oven to 375 degrees (oven rack set to second-lowest position).
Grease a 13 x 9-inch baking dish (or larger).
For the sauce; melt butter in a medium heavy-bottomed saucepan over medium heat; add in onion and garlic and saute for 3 minutes.
Add in flour and whisk 1 minute.
Slowly add in half and half cream and cook stirring until thickened, smooth and bubbly.
Add in seasoned salt, black pepper and cayenne.
Reduce heat to low and mix in the cheddar cheese, Swiss cheese and 1/4 cup parmesan cheese; stir with a wooden spoon until the the cheese has melted and the mixture is smooth (about 1-2 minutes) remove from heat.
Layer the potatoes into the baking dish (does not have to be in an even layer).
Pour the cheese sauce over, then mix the sauce using a spatula or a wooden spoon to coat the sauce with the potatoes.
Sprinkle with 1/2 cup grated Parmesan cheese (or to taste).
Bake covered for about 30 minutes.
Remove foil and continue to bake until the top is browned and the potatoes are tender About 35-40 minutes (baking time will vary depending on the size of the potato slices).