Preheat oven to 425 degrees.
Melt butter in a saucepan.
Mix together flour, salt, mustard and pepper and stir into melted butter until smooth.
Gradually add milk and onion.
Bring to a simmer
.Cook and stir constantly for 2 minutes or until thickened.
Stir in ham, peas and eggs.
Pour into pie crust or 8-inch square or 11-inch by 7-inch by 2-inch baking dish.
Seal with a top crust and flute edges; cut slits in the top.Bake for about 25 minutes or until crust is golden brown and filling is bubbly.