Prepare the tongue: Place the tongue into a large pot and completely cover it with water. Bring the water to a boil, skimming the impurities after they come to the surface until the liquid is clear. Add the vegetables and ½ teaspoon of salt, bring back to boil, then lower the heat to a simmer. Cook the tongue slowly with a partially covered lid until it can be easily pierced with a fork, about 2 hours. Remove the tongue from the pot and place on a plate to cool until you can handle it. Reserve the liquid one the side and allow it to cool. Using a knife, make an incision down the center of the tongue and start pulling the skin away to remove it completely. If it is stuck at certain parts, carefully slice it off with a paring knife. Discard the skin. Slice the tongue into ¼ inch slices on a bias. 

Make the sauce: In a large deep pan, saute the chopped onion in 2 tablespoons of vegetable oil until soft but not brown, about 10-12 minutes. Add the flour and mix it in with onions, cook for 4 minutes. Slowly add 2 cups of the reserved cooking liquid, mixing as you are pouring it to prevent lumps from forming. Add the sugar, lemon juice and raisins, season with 1 teaspoon salt. Place the sliced tongue into the sauce and stir to cover the meat with sauce. Braise the tongue over fairly low heat, partially covered, so it is simmering slowly. Allow the tongue to cook for about 1 hour 45 minutes to make sure the tongue is very tender and has absorbed the flavors of the sauce.