Heat the oil in a heavy-based saucepan and cook the onion over a high heat, stirring, for 1 minute, then stir in the curry paste and lentils. Pour in the coconut milk, then fill the can with water and add to the lentils. Simmer briskly, uncovered, for 8–9 minutes until the lentils are tender and the mixture is thick and pulpy.
Meanwhile, lightly toast the naan breads under a preheated high grill until warm and golden. Cut into fingers and serve alongside the dahl for dipping.