Prepare the kidneys: if not already cleaned by the butcher, remove the membranes and inner parts. Rinse under cold water and chop into roughly 3-4 cm pieces.
Peel the 3 onions and roughly chop them. Heat roughly 2-3 tablespoons of oil in a large pot or heavy-bottomed pan over medium-high heat. Add the kidneys and onions, cook for about 5 minutes, stirring occasionally, until lightly browned. Season with salt and pepper. Pour in the hot water (0.25 liters), then add the bay leaf and 3 cloves. Reduce heat to low and simmer for 20 minutes, until the pork reaches 63°C (145°F) internally.
Mix roughly 1 tablespoon of flour with roughly 2-3 tablespoons of cold water to form a smooth slurry. Stir this into the simmering sauce and cook for 2 minutes more until thickened. Taste and adjust seasoning with salt, pepper, and vinegar to your preference.