This is not a precise recipe, you can adjust ingredients as per your taste - less spicy, adjust oil based on how much fat there is on the meat. You could add a bit of water, but I've found that this is not necessary at all - the moisture in the onions and fat in the meat is sufficient to cook it. Try to get a mix of fatty and lean pieces of mutton.
The key ratio is 1 kg meat = half kg onion, this provides enough moisture to cook the meat. The spice mix is also something you can tune to your liking.
Process:
Either make the masala listed below, or buy ready made "meat masala" which also works.