1. Preheat the oven to 400°. Lightly grease a 10-inch cast iron skillet with a little shortening.
2. Make the filling: In a large saucepan, combine the blueberries, granulated sugar, and 1/4 cup water. Bring to a simmer over medium heat, stirring occasionally.
3. Whisk together the cornstarch and remaining 2/3 cup water in a small bowl until smooth. Stir the cornstarch mixture into the hot berries. Gently stir in the lemon zest and peaches, being careful not to mash the peaches. Reduce the heat to low and continue simmering the fruit, gently stirring, until the juices have thickened and the mixture is clear. Remove the saucepan from the heat and scoop the mixture into the prepared skillet.
4. Make the topping: Stir together the flour, oats, brown sugar, and nutmeg in a small bowl. Add the melted margarine, stirring until incorporated. Using your fingertips, work the margarine into the flour mixture, squeezing until nice and crumbly. Sprinkle the topping over the blueberry filling.
5. Bake the pie just until the topping is nicely browned, 30 to 40 minutes. Serve with ice cream, if you like.
For an all-blueberry version, omit the peaches and increase the blueberries to 6 cups.