In large saucepan, melt butter over medium heat; cook onions, stirring occasionally, for 1 minute or until softened. Sprinkle with flour; cook, stirring, for 1 minute; without browning. Gradually whisk in stock and milk; bring to boil.
Reduce heat and cook, stirring, for 5 minutes or until thickened. Stir in salt, mustard, pepper and thyme. Add frozen vegetables, chicken and potatoes. Pour filling into 1 of the pie shells.
Moisten edge with some of the egg; invert remaining pie shell over top and remove foil plate. Flute edges to seal. Brush top with egg; cut steam vents in top. Bake in 400 degree oven for about 30 minutes or until filling is bubbly and crust is golden.