Dad's Famous Potatoes - Hey Recipe

Ingredients

• butter or oil (to taste, for greasing the baking dish)
• 3 pounds (6-7 medium) Yukon Gold potatoes (thinly sliced into 1/8-inch rounds)
• 1/2 white or yellow onion (cut into slices)
• 2 tablespoons salted butter
• 1/4 cup all-purpose flour
• 1 1/2 cups milk (may substitute unsweetened almond milk or skim)
• 1 (8-ounce) package sharp cheddar cheese (shredded)
• 1/2 teaspoon garlic powder
• 3/4 teaspoon salt (plus more, to taste)
• freshly ground black pepper (to taste)
• 1/2 cup Gruyère cheese
• 1/4 cup parmesan (grated)
• fresh parsley (optional, to taste, chopped, for garnish)