Chicken stuffed with onion, mushroom and hazelnuts

I have adapted this from a recipe I found in the Mark's Daily Apple recipe book.  It's one of our favourite chicken recipes.

4 chicken breasts
1 onion
Mushrooms
1 oz hazelnuts
Chicken stock
1 tsp mustard
Sage

Cook onion in some coconut milk.  Add mushrooms and cook for about 5 mins.  Add the hazelnuts
Stuff the chicken breasts with mushroom mixture
Fry both sides of chicken and transfer to an oven-proof dish. 
Add some chicken stock to pan, 1 tsp mustard and some sage.  Cook for a couple of minutes.
Add to chicken and bake at 400F for 45 mins