Scalloped (Sliced) Potatoes with Gruyere and Thyme - A Duck's Oven

Ingredients

• 4 lbs Yukon gold potatoes
• 2 tbsp butter (plus extra for greasing the dish)
• 1 small white onion (sliced)
• 4 garlic cloves (minced)
• 6 sage leaves (torn)
• 1/4 cup flour
• 1 cup chicken or vegetable stock
• 1 cup milk (1% or 2%)
• 1 cup heavy cream
• 1 tsp kosher salt
• 1/2 tsp freshly cracked black pepper
• 1 tsp minced fresh thyme (divided)
• 1 tsp minced fresh rosemary
• 2 cups grated gruyere cheese
• 1/4 cup grated Parmesan