Preheat oven to 350 degrees F (175 degrees C). Fit the pie crust into a 9-inch pie dish.
Melt the butter in a large skillet over medium-low heat, and cook, stirring often, until the onion slices are slightly browned at the edges, about 15 minutes. Allow to cool, and arrange the onion slices into the bottom of the pie crust.
In a bowl, beat the eggs thoroughly with milk, salt, and about 1 1/4 cup of sour cream. Mix the remaining 1/4 cup of sour cream with flour in a small bowl until thoroughly combined, and whisk the flour mixture into the egg-sour cream mixture. Pour the mixture over the onions in the pie crust.
Bake in the preheated oven until set, about 45 minutes.