Slice up red onions thinly. Heat up oil in heavy bottom pot. Fry onions until golden brown, but not burnt. Remove from oil and drain.
To the fried onions, add 1-2 tbs water and peeled garlic cloves. Use a stick-blender to blend mixture into a paste.
Rub garlic-onion paste, yogurt, red chilli powder, garam masala, turmeric powder, coriander powder, cumin powder, ginger powder, salt and chopped coriander leaves into mutton. [See Note 1]
Use the mutton straight away. Heat up a tbs of ghee in Dutch oven. Add marinated mutton. Add about half cup water, then cover with lid and cook on low flame for 45min-1 hour. The amount of time would depend on the cut of your meat, so adjust accordingly.
Remove lid. Simmer further on low heat if curry is not thick enough. Taste for seasoning and add more salt if necessary.