Heat the olive oil in a large pot over medium heat. Add the diced onion and cook until it softens and turns translucent.
Stir in the minced garlic and grated ginger. Cook for one minute until the mixture smells fragrant.
Add the cumin, turmeric, and chili powder. Stir for 30 seconds to toast the spices.
Pour in the diced tomatoes with their juices, the rinsed red lentils, vegetable broth, and salt. Stir to combine.
Bring the pot to a boil, then lower the heat and simmer uncovered for 20 to 25 minutes. Stir occasionally until the lentils are tender and the soup has thickened.
Ladle the soup into bowls. Top each serving with chopped cilantro and a squeeze of lemon.