Heat the olive oil in a large pot over medium heat. Add the diced onion and cook for 4 minutes until it softens.
Stir in the minced garlic and cumin. Cook for 1 minute until fragrant.
Add the rinsed red lentils, diced tomatoes, coconut milk, and vegetable broth. Stir to combine.
Bring the mixture to a simmer, then reduce the heat to low. Cover and cook for 20 minutes, stirring once or twice, until the lentils are tender.
Turn off the heat and stir in the fresh spinach until it wilts. Season with salt and black pepper to taste.