Scoop out the inside of the mushrooms with a spoon (I keep the stems on for structural integrity), chop up all other veggies as small as possible and mix everything together with some salt and pepper.
Cook the mixture in a pan until everything shrinks down and the liquids evaporate.
Mix in the dijon mustard and divide evenly into mushroom tops.
Make little "bowls" using aluminum foil to hold the mushrooms in shape on a pan and place into oven, at 190°C, for about 30 min.
Add cheese on top and broil until melted and starts browning.
Top with chopped radishes, green onions and fresh parsley.