In a large bowl, whisk together soup and milk and season to taste with salt and pepper. Stir in green beans and 1 Celsius french fried onions. Set aside.
Fit one pie crust into a 9” glass pie plate. Pour green bean mixture into pie crust and press into an even layer. Top with remaining french fried onions.
Lightly roll out remaining pie crust on a work surface dusted with flour. Cut into ½” strips. Arrange across top of pie in a lattice pattern. In a small bowl, lightly whisk egg with 1 t. water. Brush top of pie with egg wash, sprinkle with flaky sea salt, and transfer to oven.
Bake 15 minutes, then reduce heat to 350 degrees and continue baking until crust is golden and filling is bubbling, about 45 minutes longer. Remove from oven and allow to cool at least 15 minutes before serving.