Preheat the oven to 400 degrees F (200 degrees C).
Place pie shell into a 10-inch pie plate. Combine eggs, rhubarb, white sugar, milk, cornstarch, nutmeg, and salt in a bowl. Pour mixture into pie shell.
Blend flour, brown sugar, and margarine in a bowl using an electric mixer until combined. Sprinkle mixture on top of pie.
Bake in the preheated oven for 15 minutes. Remove from oven; reduce oven temperature to 350 degrees F (175 degrees C). Return to oven and continue to bake until set and golden brown, about 40 minutes more.