Potato Galette

Ingredients (12)

• 2 1/2 pounds Yukon Gold potatoes
• 5 ounces (10 tablespoons) salted butter, melted, divided
• 2 medium onions, sliced
• 1 1/2 teaspoons kosher salt, divided
• 6 cloves garlic, roasted, minced
• 1 tablespoon cornstarch
• 1/2 teaspoon garlic powder
• 1/2 teaspoon freshly ground black pepper
• 1 cup grated Asiago cheese
• 1/4 cup grated Parmesan cheese, plus more for garnish
• Fresh rosemary, for garnish
• Sour cream, for garnish