Mushroom and Potato Gratin

Bahan-bahan (8 porsi)

• 1 tablespoon extra virgin olive oil
• 1 tablespoon unsalted butter
• 1 pound (5 to 6 cups) mixed fresh mushrooms ((shiitakes, cremini, white), washed, trimmed & sliced ¼-inch thick)
• 2 medium garlic cloves, (minced)
• ½ teaspoon kosher salt
• 2 tablespoons chopped fresh flat-leaf parsley
• freshly ground black pepper
• 2 to 3 tablespoons heavy whipping cream, broth or lemon juice
• additional chopped herbs ((optional))
• 1 tablespoon unsalted butter, (softened at room temperature)
• 1 cup low-sodium chicken broth
• 1 cup heavy whipping cream
• 2½ pounds russet potatoes
• 1 tablespoon chopped fresh thyme
• 2½ teaspoons kosher salt
• freshly ground black pepper
• ½ cup (about 2-ounces) finely grated Gruyère cheese