1. Prepare the Lamb Shanks: 

Season the American Lamb shanks with salt and pepper. Heat the vegetable oil in a large skillet or Dutch oven over medium-high heat. Brown the lamb shanks on all sides, working in batches if necessary. Remove the lamb shanks from the skillet and set aside. 

2. Saute Aromatics: 

In the same skillet, add the chopped onions and cook until they are soft and translucent, about 5 minutes. Add the minced garlic and ginger, and cook for another 2 minutes until fragrant. 

3. Add the Spices: 

Add the curry powder, ground cumin, ground coriander, turmeric, cayenne pepper, and tomato paste to the skillet. Stir well to combine and cook for another 2 minutes to toast the spices. 

4. Add Tomatoes and Liquid: 

Stir in the chopped tomatoes and cook for 3-4 minutes until they begin to soften. Pour in the beef or vegetable broth and coconut milk, stirring to combine. 

5. Simmer the Curry: 

Return the lamb shanks to the skillet, nestling them into the sauce. Bring the mixture to a simmer, then reduce the heat to low. Cover the skillet or Dutch oven and let it simmer gently for about 1.5 hours. Then remove the lid and continue to simmer until the lamb is tender and falling off the bone. Turn the shanks over a couple of times in the cooking process to ensure they cook evenly and do not dry out. Alternatively you can cook the curry in the oven for 2.5 hours at 300 degrees.