Drain the water and transfer to a large plate. Knead with the help of a fork, it doesn't need to be homogeneous, you can leave some whole grains to give the hamburger texture;
Chop the onion into cubes and brown in vegetable oil. To make less dishes dirty, I suggest you brown the onion in the same pan used to cook the lentils;
Mix the mashed lentil with the golden onion, chopped parsley, salt to taste and oatmeal (or other flour); if you want to add olives and cilantro that is also great.
Model the hamburgers in the size and thickness you want;
Heat a drizzle of vegetable oil and fry the hamburger on both sides until golden. The hamburgers you are not going to eat, freeze them without frying for the next few days.