Rinse lentils in water, and remove dust and debris. No need to soak lentils prior cooking.
In a large saucepan add the lentils, whole onion (or a quarter of a large onion), minced garlic, bay leaves, one piece of lemon rind (whole, not grated!), salt, pepper, and water. Bring to boil, and let it simmer covered for about 15-20 minutes, until lentils are tender.
In the meantime in a small bowl whisk together the flour, milk and mustard.
Once the lentils are tender, remove and discard the onion, lemon rind, and bay leaves.
Add 3-5 tbsp of hot water from the lentils to the flour mix and stir. Remove the lentils from the heat, let them cool for a few minutes, add the flour-mustard mixture and stir. Put the saucepan back on medium heat, and cook it for another 3-4 more minutes while stirring. Add the lemon juice or vinegar, and more salt, pepper, mustard if needed.
Garnish with a hard boiled egg and/or wieners and fresh bread for a hearty Hungarian meal!