Cook the lentils al dente in 10-15 minutes. Drain the lentils. Cook the sweet potato al dente and leave to cool. Roughly grate the sweet potato.
Mix the lentils, sweet potato with the parsley, cumin, paprika powder, egg and flour. Add additional flour if the mixture is too sticky. Shape the mixture into small burger patties.
Cut the onion into half rings and the avocado into slices. Fry the burgers on both sides until golden brown. Meanwhile, sauté the onion in a tablespoon of olive oil.
Top the bun with the patties, onion and avocado. Enjoy your meal!