One way you can do is by grinding onions (u need a lot of them), and mix it with the ingredients(chicken or vegetables)  or when u go to buy keema u can ask them to give u some fats(goat's are usually good)...
this will make momo juicy...yum,,,,,

as abt the achar,,was it sesame (teel) ko achar....i am not sure what kind of achar u are talking abt..

happy eating MOMO