Preheat the oven to 350 degrees F.

Heat the butter in a large saucepan over medium-high heat and add the onions and cook until soft and translucent, 5-6 minutes. Add the garlic and cook for another minute. Add the flour and stir to combine. Cook for a minute or two. Add the milk and cream, stirring continually to prevent clumping. Slowly bring to a boil, reduce the heat and simmer until slightly thickened.

Add the cheddar cheese, parmesan cheese and salt and stir until melted. Add the potatoes and stir until thoroughly combined.

Pour the mixture into a 2 quart casserole dish. Sprinkle with the remaining Parmesan cheese. Cover with foil. Bake for 1 hour or until the potatoes are tender (time can vary depending on how thickly the potatoes are sliced).

Uncover and bake for another 20 minutes or until the top is lightly browned and to enable excess moisture to evaporate.

Let sit for about 5 minutes before serving. Sprinkle with some chopped parsley for garnish if desired.