In a spice grinder, combine 2/3 tsp of black peppercorns, 1 tsp coriander seeds and 1 tsp ground cardamom and grind to a fine powder. If you don't own a spice grinder, swap for powdered coriander and freshly ground black pepper from a mill.
In a large casserole, heat 3 tbsp of cooking oil to high. Add pieces of lamb, making sure there is enough space between them, and sear from all sides. Use an oil splatter shield! Work in batches to not overcrowd the pan. Typically, you will need 3 goes. Transfer the seared meat to a bowl.
Add 2 more tbsp of cooking oil to the casserole and briefly fry onions and garlic until the aromas develop.
Add the tomatoes and season with 1/2 tsp salt to help them release the juices.
Add watermelon and stir to combine.
Cover and cook for 8-10 minutes over low heat
Return the meat back to the casserole. Season with the spice mixture and another tea spoon of salt.
Cover and simmer for 45 minutes over the lowest heat.
By that time, the meat should be tender. Taste and cook for a few more minutes if necessary.
Add green pumpkin and 6-8 mint leaves and give the stew a good mix.
Continue cooking for 8-10 further minutes until the pumpkin has softened but is not mushy.
Taste the sauce and adjust for salt, if required.
Serve garnished with small watermelon cubes and some fresh mint leaves.