Warm a stock pot over medium high heat and coat with olive oil.
Add the onions and ginger and reduce the heat to medium. Cook, stirring, until onions are translucent and are starting to brown. Mix in the cumin seeds, stir, then add the tomato paste. Stir to incorporate. Add the lentils and broth and simmer for 5 minutes.
Add the beets and carrots, and more liquid if needed (to achieve a brothy stew consistency.) Simmer, covered, over low heat for 25 minutes or until both lentils and vegetables are tender.
Stir in the spinach and tomatoes, and season to taste with salt and pepper.
Add the lemon juice or vinegar, and a drizzle of olive oil.
