In a bowl, combine 1 ¼ cups coconut milk, 5 tsp garlic, 5 tsp ginger, the cumin, ground coriander and a pinch of salt. Whisk well to combine. Add ground lamb and cover; refrigerate 2 hours, up to overnight.
At dinner time:
Peel and dice onion. Heat coconut oil in a skillet over medium-high. Add onion and cook 5 minutes, stirring regularly, until golden. Add remaining 5 tsp garlic, 2 ½ tsp ginger, the tomato paste, garam masala and cayenne. Simmer 5 minutes.
Dice tomato. Add to the sauce, along with remaining 1 ¼ cups coconut milk and a pinch of salt and pepper. Bring to a boil, then reduce to a simmer and cover. Cook 10 minutes.
Meanwhile, remove lamb from fridge. Heat a separate skillet over medium-high. Add ground lamb and its juices. Cook, stirring occasionally, 8 minutes, or until cooked through.
Add sauce to the lamb mixture and simmer uncovered 2 minutes. Transfer to a bowl and garnish with fresh cilantro if desired.