Heat the vegetable 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 lentils, diced tomatoes with their juice, and vegetable broth. Bring the mixture to a boil, then lower the heat and simmer uncovered for 20 minutes.
Stir in the peanut butter until it melts into the broth. Continue simmering for 10 more minutes until the lentils are tender.
Add the chopped kale and cook for 5 minutes until it wilts. Season with salt and black pepper to taste before serving.