Heat the olive oil in a large pot over medium heat. Add the diced onion and cook for 3 minutes until it softens.
Stir in the minced garlic and Thai red curry paste. Cook for 1 minute until the paste becomes fragrant.
Add the sweet potato cubes and rinsed red lentils to the pot. Stir to coat everything with the curry mixture.
Pour in the vegetable broth and coconut milk. Bring the mixture to a boil, then lower the heat to a simmer.
Cover the pot and cook for 20 minutes, stirring once or twice, until the sweet potato is tender and the lentils have broken down slightly.
Stir in the fresh spinach and cook for 2 more minutes until it wilts. Season with salt and black pepper.
Ladle the stew into bowls and top each serving with cilantro and a squeeze of lime.