1. Open the chicken breasts on the top and on the sides, then refrigerate them.
2. Slice the onion.
3. Thoroughly clean the button mushrooms and cut them into small cubes.
4. Coarsely chop the slices of ham.
5. Sauté the onions in a little butter until they are golden brown.
6. Season the onions with salt and pepper.
7. Add the mushrooms and let them cook for a few minutes until they release their water.
8. Deglaze with a little white wine and scrape off the browned bits.
9. Once the wine has evaporated, add the ham and let it brown for a few minutes.
10. Add a little cream and mustard to bind the stuffing with some fresh herbs.
11. Cut a few sticks of cheese of your choice and add them into the chicken breasts.
12. Stuff the chicken breasts with the filling and sprinkle some grated cheese on top.
13. Bake at 200°C for 20 minutes.