Heat oil and butter in a hot pan.When butter is melted but not brown add thinly sliced mushrooms and garlic cook for 3-5 minutes, add thinly sliced capsicum and onions cook for another 3-5 minutes. Add cayenne pepper,salt and black pepper.Stir and cook for about 2 minutes. Allow to cool.
Take each chicken breast and slice down one side but not the whole way through.This is to make a pocket.Halve the mushroom mixture and gently place mixture in each pocket not quite filling them.
Refrigerate and then either pan fry in olive oil,grill,BBQ or oven bake.Depending on the size of the breast cooking should take about 15 minutes.Serve with your favourite side dishes.