Cheesy Baked Scalloped Potatoes

Ingredients

• 4 pounds Yukon Gold potatoes (sliced 1/4 inch)
• 4 tablespoons butter
• 1 small onion (thinly sliced)
• 6 cloves garlic (finely chopped)
• 4 tablespoons all-purpose flour
• 2 cups whole milk
• 1 cup chicken broth
• 1 1/2 teaspoons salt
• 1/2 teaspoon black pepper
• 2 teaspoons fresh thyme (divided)
• 16 oz sharp cheddar cheese (divided (shredded))
• 3/4 cup parmesan cheese (divided (grated))
• chives (parsley, or green onions for garnish)