Ingredient:

• Pastry for single-crust pie (9 inches)
• 1/2 cup quick-cooking oats
• 1/3 cup all-purpose flour
• 1/3 cup packed brown sugar
• 1/2 teaspoon grated orange or lemon peel
• 1/3 cup cold butter, cubed
• 1 cup sugar
• 3 tablespoons cornstarch
• 1/2 teaspoon ground cinnamon
• 1/4 teaspoon ground nutmeg
• 1 cup sour cream
• 1 large egg, lightly beaten
• 3 cups chopped fresh or frozen rhubarb