Heat the olive oil in a large pot over medium heat. Add the chopped onion and minced garlic. Cook for 3 minutes until the onion softens.
Stir in the ground cumin, turmeric, and chili powder. Cook for 1 minute to toast the spices.
Add the rinsed red lentils and vegetable broth. Bring the mixture to a boil, then reduce to a simmer. Cook uncovered for 15 minutes, stirring now and then.
Stir in the diced tomatoes. Continue simmering for 10 more minutes until the lentils break down and the stew thickens.
Add the fresh spinach and stir until it wilts, about 2 minutes. Taste and add salt as needed.
Ladle the stew into bowls. Top each serving with chopped cilantro and a squeeze of lemon.