Lentils cooked Czech Style

Clean one pound of lentils, removing rocks and other non lentils chaff. Put the lentils in a bowl and cover with water. Allow to soak for several hours. Drain and add fresh water. Boil until soft. Chop an onion into small pieces and fry in a separate pan until golden brown. Add some flour and a small amount of water, making a yellowish roux. Add a bay leaf and some thyme. Boil for about 10 minutes and add to cooked lentils. Season with vinegar or lemon juice. Add salt, pepper and sugar. Cook over low heat for about 10 to 15 minutes.