Preheat oven to 425˚F. Prepare the pie crust in a 9-inch pie pan and let it chill in the refrigerator until ready to use.
To make the custard filling, whisk together sugar, flour, and nutmeg in a large mixing bowl. Add sour cream, milk, egg, egg yolks, vanilla extract and mix well.
Put sliced peaches in a prepared pie pan. Pour the custard filling over peaches distributing evenly. Bake at 425˚F for 15 minutes. Lower the oven temperature to 375˚F and bake another 10 minutes.
Meanwhile, make the streusel topping. Whisk flour, sugar, and cinnamon together. Add butter pieces and rub into the flour mixture until you get coarse crumbs.
Take the pie out of oven and sprinkle over to the half baked custard. Continue to bake for 25-30 minutes until the stresuel is golden brown and the custard is bubbly.
Cool your pie on a wired rack for at least 3 hours before you serve. Serve at room temperature or chilled.