Heat the oil in a large pot over medium heat. Add the onion and cook for 3 minutes until it softens.
Stir in the garlic, ginger, and red curry paste. Cook for 1 minute until fragrant.
Add the mushrooms and cook for 4 minutes, stirring now and then, until they release some liquid.
Pour in the lentils and vegetable broth. Bring to a simmer, then cover and cook for 20 minutes.
Stir in the coconut milk and soy sauce. Simmer uncovered for 10 more minutes until the lentils are tender.
Remove from heat. Stir in the lime juice and Thai basil leaves. Taste and add salt and pepper as needed.