Place the split peas, onion, tomatoes, ginger, ground cumin, turmeric, crushed garlic, curry leaves and stock into a slow cooker. Add most of the chili and stir to combine. Cook on high for four hours, until the split peas are tender.
Season the dal generously with salt and freshly ground black pepper. Just before serving, heat the coconut oil in a frying pan. When the oil is very hot, add the whole cumin seeds and the sliced garlic. Fry until the garlic is golden-brown and the cumin smells toasty and almost smoky. Spoon the hot spiced oil over the dhal, scatter with the remaining green chili, then serve with lemon wedges for squeezing.