Adding masalas in the mutton will help. In India, we add onion, garlic, ginger and garam masalas --- a combination of black and green cardamoms, cloves, cinnamon, black pepper etc., to the mutton, while cooking it. As a result the unfavorable smell is gone and it tastes great.
I have decided to do some mutton for tomorrow and wanted to cook it slow overnight. I was planning on sticking it in the oven and thought I would see if anyone here has any suggestions or tricks for making it tasty!