Preheat oven 375 degrees
Butter casserole dish generously with butter, cooking spray or olive oil and set aside.
Peel potatoes and thinly slice. After peeling them set them in a large bowl of cold water as this helps remove some starch. Drain and rinse one more time.
Chop onions and mince garlic and place in small bowl. I also put the spices in a bowl to combine.
Meanwhile to make cheesy sauce melt butter over medium heat in sauce pan. Stir in cornstarch, garlic powder, ground mustard, and papirka to form roux.
Stir in half & half cream and sour cream and whisk about 8-10 minutes till starting to thicken.
Turn off heat add cheese reserving 1 cup of cheddar and parmesan cheese for topping.
Place half of the sliced potatoes on bottom of casserole dish.
Sprinkle with half the chopped onion and garlic.
Top with half the cheese sauce.
Repeat layers starting with remaining potatoes.
End by topping with additional cheese.
Cover and bake in preheated oven 45 minutes.
Remove from oven and carefully remove cover and continue to bake another 15 minutes.
Allow at least 10 minutes to cool prior to serving.