Rinse the lentils, put them in a pot with the water, the whole onion, the peeled carrot and parsley root, salt lightly, and cook over medium heat for about 35-40 minutes, until the lentils are completely tender.
Meanwhile make the pastry: quickly rub the cold, cubed butter into the flour with a pinch of salt, add enough ice water for it to just come together; fold and roll out a few times, then wrap in plastic and chill for at least 20 minutes.
Remove the vegetables and the onion from the tender lentils, and purée the lentils with a sieve or an immersion blender.
In a separate pan, fry the flour in the lard until red, remove from the heat, stir in the paprika, and immediately add 100-200 ml of the lentil cooking water to make a smooth roux.
Stir the roux into the puréed lentils, bring to the boil, season with salt, then stir in the sour cream.
Preheat the oven to 190°C. Roll out the chilled pastry, and line a buttered tart pan with it, leaving the edge a little higher than the pan.
Pour in the lentil purée, smooth it out, and bake for 35-40 minutes, until the pastry is golden brown.
Slice and serve lukewarm or warm.
Based on: Malatinszky Fanny szakácskönyve (a classic Hungarian cookbook), modernised. Calories and protein are estimated.