Honestly, this is a great recipe, I make it all the while for guests and it always turns out lovely. The lady who's recipe book I took it from uses the concept 'take a classic dish, consider how it is made, then test, modify and fine tune all the different possibilities to create the ultimate version. She was told to be prepared to test each recipe not once, not twice but possibly a dozen times, whatever it took to achieve the best.
My tips would be to make sure that the lamb is marinated well and still lovely and tender but still moist after the long simmering in the onions. Lastly, put in the oven just before you are going to use it otherwise if you heat it and then reheat later on, it goes a little dry.
I'll be interested how you get on with it.