Preheat oven to 350°.

Using a mandolin, thinly slice potatoes until ⅛ in thick. Set aside.

In a small saucepan, melt butter over medium-low heat. Add garlic and cook for 2 minutes, stirring frequently.

Add in salt, pepper, and rosemary to the saucepan and whisk. Slowly add in milk and whisk until everything is combined.

Remove saucepan from heat and slowly stir in ½ cup of the cheddar cheese.

In a circular greased baking dish, line potatoes in a circular ring. Once the outer ring is complete, continue with inner rings until the whole dish is filled.

Pour cheese sauce evenly over potatoes. Top dish with remaining Cheddar cheese and Parmesan cheese.

Bake for one hour, or until cheese is browned and bubbly.