Pick over and rinse the lentils, put them in cold water, and cook, covered, until tender, about 40-45 minutes.
Salt it once it's nearly tender.
Fry the onion in the lard until light brown, sprinkle with the flour, fry a little more, then make a roux by adding a ladleful of the lentils' cooking liquid.
Stir the roux into the lentils, bring to a boil, then purée smooth with an immersion blender or push through a sieve.
Season with vinegar, thin with water to the desired consistency if needed.
Serve with toasted bread cubes.
Based on: Malatinszky Fanny szakácskönyve (a classic Hungarian cookbook), modernised. Calories and protein are estimated.