Chop the onion, ham and cheese into small pieces. Put all ingredients in a mixing bowl and stir well. In the original, this should actually be done by shaking, but stirring everything mixes 100% - which doesn`t always have to be the case when shaking
Then put everything in a greased baking tin (or, for larger quantities, directly on the baking sheet lined with baking paper). Now bake everything at 180 ° C (top / bottom heat) for about 40 minutes.