Bilad al-Sham Kitchen - Mushroom-Stuffed Chicken

Stuffed Chicken and Mushrooms

4 chicken breasts

1 tablespoon butter

1 tablespoon finely chopped onion

2 garlic cloves

250g mushrooms

Lemon juice

Salt and pepper

Puff pastry dough

1 egg

2 tablespoons mustard

3/4 cup chicken stock

3/4 cup cream

Chives for garnish