Reduce the oven temperature to 350 degrees F.
Add 1¼ cups granulated sugar and the all-purpose flour to a small saucepan and whisk to combine.
Separate the eggs, placing the whites in the bowl of a stand mixer fitted with the whip attachment. Set aside until its time to make the meringue
Add the three egg yolks and sour cream to the sugar and flour mixture. Whisk to combine.
Add the raisins and stir to combine.
Cook over medium heat, stirring constantly, until the mixture is thick and bubbly. Turn off the heat, and leave the mixture on the burner to keep it warm.
Add the cream of tartar to the egg whites and whip on medium-high until really frothy.
Slowly add the remaining half cup of sugar, one tablespoon at a time, while continuously whipping the egg whites.
When the sugar has been added, turn the mixer to high speed and beat until the meringue is glossy and holds stiff peaks.
Spread the warm raisin filling mixture into the pre-baked pie crust. Smooth the top with a rubber spatula.
Spoon meringue over the top of the filling, spreading it to the edges of the pie to seal the pie.*Don't leave any of the filling showing. Place the pan on the middle rack of the preheated oven and bake for 15 minutes.
Place the pan on a wire rack to cool for one hour.
Refrigerate the cooled pie for 3 - 4 hours before serving.
Serve chilled or at room temperature.