For the tamarind sauce: Put the coriander seeds and cinnamon stick into a saucepan and toast over medium heat for 1 minute. Stir in the tamarind, coconut milk, grapefruit juice, water, sugar, ginger, salt, and pepper to taste, breaking up the tamarind with a spoon. Bring the ingredients to a boil, and then reduce the mixture to a simmer. Cook until the sauce has thickened, about 15 to 20 minutes. Strain the sauce into a bowl, and adjust the seasonings with salt, and black pepper, to taste.
For the marinade: Combine the shallot, ginger, garlic, oil, and black pepper in a bowl, and mix.
Add the lamb pieces to the mixture and toss to coat evenly. Cover the bowl with plastic wrap and place in the refrigerator to marinate for a minimum of 30 minutes and a maximum of 24 hours.
For the kebabs: Preheat a grill to medium-high and lightly oil the grates.
Assemble the kebabs by threading 1 piece of red onion onto a wooden skewer that has been soaked in water, followed by 1 piece of lamb through the same skewer, so the meat is resting on the red onion. Skewer the mint stems and leaves, wrapping them around the red onion and the lamb. Repeat with the remaining ingredients.
Transfer the kebabs to a serving platter and serve with the Tamarind Sauce and the Black Eyed Peas and Rice.
Add the black eyed peas to the pot 5 minutes before the end of the cooking process.
Transfer the finished dish to a bowl, let it cool slightly, and garnish with the mint.