Combine the lentils with 2 ½ cups (600ml) of the water in a small saucepan over medium heat. Cook the lentils for 13 to 15 minutes, or until they are al dente, stirring once halfway through the cooking time. Drain the lentils and set aside.
Meanwhile, heat the oil in a large saucepan over medium heat. Add the onion, garlic, and a pinch salt and cook until the onion is translucent, about 4 minutes. Add the ginger and red curry paste and stir to combine. Cook the mixture for 1 to 2 minutes, until the paste is fragrant. Add the bell pepper and a splash of water and cook for 5 minutes.
Add the milk, remaining ¾ cup (180ml) water, salt, sugar, cayenne, lime zest, and lime juice and stir to combine. Add the cooked lentils and bring the mixture to a rolling boil, 6 to 7 minutes. Taste and adjust the seasonings. Reduce the heat to low. Add the Thai basil and 2 tablespoons (6g) of the cilantro (if using) and stir to combine. Cover the saucepan and simmer for a few minutes, until the vegetables are cooked through and the flavors meld. Garnish with the remaining cilantro and lemon wedges and serve.
Variations: Add some smooth peanut butter and curry powder for a Massaman-style soup. Add 10 ounces (284g) cubed firm or extra-firm tofu instead of cooked lentils.