Heat the olive 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, and cayenne pepper. Cook for 1 minute until the spices smell fragrant.
Add the rinsed red lentils, vegetable broth, and diced tomatoes. Bring the mixture to a boil.
Reduce the heat and simmer for 20 minutes, stirring now and then, until the lentils are tender.
Turn off the heat and stir in the chopped arugula until it wilts. Add the lemon juice, then season with salt and black pepper.