Scalloped Potatoes

Ingredients

• 3 tablespoons butter
• 1 yellow onion (or small white, peeled and thinly sliced)
• 4 large garlic cloves (minced)
• 3 tablespoons flour (all-purpose)
• 1 cup chicken stock
• 2 cups milk (I used 2% low-fat milk)
• 1 teaspoon kosher salt
• 1/2 teaspoon black pepper
• 2 teaspoons fresh thyme leaves (divided)
• 4 pounds yukon gold potatoes (sliced into 1/8-inch rounds, and peeled beforehand, if desired)
• 1 1/2 cups sharp cheddar cheese (freshly-grated, divided)
• 1/2 cup grated parmesan cheese (freshly-)