Soak the sweetbreads in several changes of cold water for 2 hours. Remove from the water bath and press between 2 plates.
In a large fish poacher or other pot, combine the chopped fennel, carrots, celery, 6 cloves garlic, chopped onion, wine, vinegar, mustard seed, fennel seed, celery seed, chili flakes, peppercorns, salt and water and bring to a boil. Immediately reduce to a simmer and cook 20 minutes. Add the thyme, sage and rosemary and cook 5 minutes more. Strain out the solids and return to a boil. Add the sweetbreads and return to a simmer. Cook about 10 minutes, then drain and set aside.
In a tall-sided pot, heat the olive oil to 375 degrees F.
Combine the semolina and quick mixing flour in a shallow bowl or plate. Dredge the sweetbreads, sliced onion, sliced fennel and 2 of the lemons in the flour mixture and cook in the hot oil until golden brown, working in batches if necessary to avoid overcrowding the pan. Remove the fried foods with a spider or slotted spoon, season with salt and pepper and serve with the uncooked lemon wedges on the side.
Recipe copyright 2000, Mario Batali. All Rights Reserved.