Heat the vegetable oil in a large pot over medium heat. Add the diced onion and cook until it softens, about 5 minutes.
Stir in the minced garlic, cumin, turmeric, garam masala, and cayenne pepper. Cook for 1 minute until the spices smell fragrant.
Add the rinsed red lentils, vegetable broth, and diced tomatoes with their liquid. Bring the mixture to a boil.
Reduce the heat to low, cover the pot, and simmer for 20 minutes, stirring now and then, until the lentils are tender.
Stir in the fresh spinach and cook for another 3 minutes until it wilts. Season with salt and black pepper.
Serve the stew hot with lemon wedges on the side for squeezing over each bowl.