Sweet Potato and Caramelized Onion Gratin Recipe

Ingredients

• 2 tablespoons butter
• 3 medium onions, cut into 1/4-inch slices
• 1 3/4 teaspoons salt
• 1 tablespoon balsamic vinegar
• 3 cups heavy whipping cream
• 2 sprigs fresh thyme
• 2 sprigs fresh sage
• 2 cloves garlic, crushed
• 1/2 teaspoon ground black pepper
• 2 1/2 lb sweet potatoes, peeled, cut into 1/4-inch slices
• 1 lb Yukon Gold potatoes, peeled, cut into 1/4-inch slices
• 2/3 cup grated gluten-free Parmesan cheese