Soak sweetbreads in cold water and refrigerate overnight. Drain and rinse thoroughly with cold running water.
In a large saucepan over medium heat, add butter and bacon. Add carrots, celery, onion and garlic. Cook for a few minutes until softened. Add thyme, bay leaf, and sweetbreads. Season with a little salt. Add enough cold water to cover and bring to a boil. Remove from heat and allow to sit for 10 minutes.
Remove sweetbreads and transfer to a clean container. Strain liquid and cool. Pour over sweetbreads and refrigerate until ready to use.
Preheat oven to 400 degrees Fahrenheit. Remove any membranes from the sweetbreads and cut the meat into 2 inch pieces. Season with salt and freshly ground white pepper. In skillet, heat the oil over medium high heat. When hot, add the sweetbreads and sear on all sides. Add reduction and transfer to oven for 3 – 5 minutes or until well glazed. Add remaining butter & keep warm.
Heat veal stock with sherry vinegar in small pot over medium heat. Add lemon juice and whisk to mix. Finish with olive oil & keep warm.
Melt butter with olive oil in a medium saucepan over medium heat. Add bacon, shallot, carrot, leek, celery, and garlic. Cover pan and sweat vegetables for 5 minutes.
Add bay leaves, thyme & lentils and enough stock to cover. Simmer covered, checking occasionally to ensure the lentils are covered with stock. Keep adding stock as needed until lentils are cooked, about 20 – 30 minutes. Remove pot from heat. Take out and discard vegetables, herbs, and bacon. Allow lentils to cool and refrigerate until ready to use.
To finish the lentils, heat 1 tsp of the butter in a large pan over medium heat. Sweat bacon, minced garlic and shallots until soft. Add veal stock and reduce. Add lentils and cook until lentils are warmed through. Add truffles and/or a few drops of truffle oil. Season to taste with salt and freshly ground white pepper. Add mixed herbs and remaining butter. Adjust seasoning & keep warm.
Place a mound of truffled lentils in center of 4 plates. Place sweetbreads on lentils. Spoon sauce on top and around dish.