chicken pie recipe the filling part 1 of 2

Music: As Leaves Fall
Musician: @iksonmusic
Ingredients
8 skinless chicken thighs
1 onion peeled and sliced
2 bay leaves
50g butter
2 tablespoons plain flour
1 teaspoon english mustard powder (don't lick the spoon!)
400mls chicken stock (2 stock cubes)