Ina Garten's Potato-Fennel Gratin

Ingredients (9)

• 2 small fennel bulbs
• 1 yellow onion
• 2 tablespoons good olive oil
• 1 tablespoon unsalted butter
• 2 pounds russet potatoes
• 2 cups plus 2 tablespoons heavy cream
• 2 1/2 cups grated Gruyère cheese
• 1 teaspoon kosher salt
• 1/2 teaspoon freshly ground black pepper