Gratin of Fennel, Leeks and Potatoes

Ingredients

• 2 tablespoons plus 2 teaspoons unsalted butter
• 4 leeks, white and 2" of green, 1/4" slices, washed, dried
• 3 bulbs fennel, trimmed, 1/4" slices
• 4 medium red potatoes, peeled, 1/4" slices
• salt and freshly ground pepper
• 2 cups heavy cream
• 1/2 cup milk
• 3/4 cups grated Parmesan cheese