Melt butter in a saucepan; stir in flour, salt, mustard and pepper until smooth. Gradually add milk and onion; bring to a boil. Cook and stir for 2 minutes or until thickened. Stir in ham, peas and eggs. Pour into an ungreased 8-in. square or 11x7-in. baking dish. On a floured surface, roll pastry to fit top of dish; place over filling. Seal and flute edges; cut slits in the top. Bake at 425° for 25 minutes or until crust is golden brown and filling is bubbly.