Heat olive oil in a large pot over medium heat. Add onion and cook until softened, about 5 minutes. Add garlic and cook for 1 minute more.
Add lentils, vegetable broth, cumin, paprika, and red pepper flakes (if using) to the pot. Bring to a boil, then reduce heat and simmer for 20-25 minutes, or until lentils are tender.
Stir in tomato paste and cook for 2 minutes more.
Remove from heat and stir in lemon juice. Season with salt and pepper to taste.