Slice one onion and fry until golden brown. Set aside
Cut he beef shank into large chunks
Heat the oil and then add beef shanks (not the bones) and brown on all sides
Add ginger and garlic paste and fry for 2 minutes
Add all the masala and fry for 1 minute
Add the bones
Now add water. Just enough to cover the beef and bones
Increase heat to high until it starts to boil
Turn heat to simmer and cover the pot
Let it cook for 3 hours
Open the pot and take out about 2 cups of gravy in a bowl
Stir in two table spoons of flour into the gravy bowl. Make sure to add little at a time and stir with a whisk so as not to make any flour clumps
Add the fried onions to the pot
Add the gravy with flour to the pot and stir to mix in
Simmer for 30 minutes
Serve the Nihari with Nan Bread and condiments on the side