Slice the onions into fine half rings. Cook the onions with the sunflower oil and sugar in a saucepan with a lid over low heat. Season with a little salt. Stir regularly.
Drain the chickpeas and reserve the liquid (aquafaba). Put the chickpeas, adzuki beans, lentils, oats, and nutritional yeast in a blender and mix briefly on low speed so that some texture remains. Otherwise, you'll get hummus. To achieve a nice mixture, you can stir occasionally while blending. Season with pepper and salt. Pour it into a bowl and mix with a spatula. Roll balls of about 120 g and then flatten them into burgers. Place them on a plate and keep in the fridge until you are ready to fry them.
Put the aquafaba and mustard in a blender. Blend on low speed and gradually add the sunflower oil. Add the lime juice. Season to taste with pepper and salt. Cut the iceberg lettuce into chiffonade. Cut the pickles into fine strips.
Fry the burgers in a pan with a little sunflower oil. Sprinkle extra nutritional yeast on the burger for an extra cheesy flavor. Cut the buns in half and toast the inside in a frying pan.
Finishing: Spread mayonnaise on the bottom part of the burger. Place the hamburger on top, followed by the caramelized onions, pickles, and lettuce. Finish with extra mayonnaise. Place the top of the burger bun on top. Enjoy!