Slice the chicken breast midway to create a pocket. Do not cut all the way.
Mix all the ingredients for marinade and coat the chicken with it. Let it sit for 30 minutes.
In a shallow pan, heat the oil and fry the mushrooms till they are brown.
Add the spinach and poppy seeds and mix it in.
Make sure to preheat the oven to 425 F before starting to stuff the chicken.
Stuff the chicken with the filling made earlier along with the cheese.
Insert a few toothpicks to seal the chicken so that ingredients do not come out during baking.
Bake for 20 min with a loosely covered aluminium foil. Then continue baking it for 5 min longer without the cover. This is to avoid excessive browning of the chicken.