Egg shells stuffed with mushrooms, eggs and onions mix is an appetizer I asked my mom to do for each celebration, we had, when I was little.
It is still my favorite dish; now I cook it myself when have enough time and special state of mind to stub some eggs and dice everything into little pieces 
To get empty egg shells is the most tricky part in making stuffed egg shells: you may waste one or two eggs while learning to do so, just be brave and persistent.
You are lucky if chickens, which produced eggs you are planning on using to prepare this appetizer, have had enough calcium in their diet, that will make the whole deal with getting nice looking egg shell halves much more easier. Unfortunately, it wasn’t a case for me: the shells of eggs I had were crushing when breathed on.
That didn’t do any bad for taste (believe me!), just made pictures a bit less nice looking than I expected, but I hope you will still like the recipe enough to try it out.