Cook the onion. Heat the oil in a large pot over medium heat. Add the onion and cook for 4–5 minutes, stirring occasionally, until softened.
Add garlic, ginger and curry paste. Add the garlic, ginger and red curry paste. Cook for 1–2 minutes, stirring frequently, until fragrant and the onion is well coated in the curry paste.
Add the remaining ingredients. Add the coconut milk, vegetable broth, coconut sugar, tamari or soy sauce and rinsed red lentils. Stir well to combine.
Simmer. Bring the mixture to a gentle simmer, then reduce the heat to maintain a low simmer. Cook partially covered for 15–20 minutes, stirring occasionally, until the lentils are tender and the curry is thick and creamy. If the curry becomes too thick, stir in a splash or water or broth to adjust the consistency to your preference.
Finish with lime. Remove from the heat and stir in fresh lime juice. Taste and adjust the seasoning as needed.
Serve. Serve warm on its own or over cooked rice or quinoa. Top with fresh cilantro, Thai basil or green onion, if desired.