Heat the olive oil in a large pot over medium heat. Add the onion and cook until it softens.
Stir in the garlic and ginger. Cook for one minute until fragrant.
Add the cumin, turmeric, and curry powder. Stir for 30 seconds to toast the spices.
Pour in the red lentils, vegetable broth, and diced tomatoes. Bring the mixture to a boil.
Reduce the heat and simmer for 20 minutes, stirring occasionally, until the lentils are tender.
Stir in the spinach and cook until it wilts. Season with salt and pepper.
Remove the pot from the heat and stir in the lemon juice before serving.