I’ve never cooked Mongolian Beef at home but when I’m looking for a recipe I always check the allrecipes site and see what’s over there - I especially like to read the comments under recipes to check out ideas for adjustments, techniques and additions to recipes.
If no one here has a recipe - give this site a shot:
http://allrecipes.com/Search/Recipes.aspx?WithTerm=mongolian%20beef
Thank you