Preheat oven to 350 degrees. Season tenderloin with porcini powder, salt and pepper. Tie meat at one inch intervals if desired. Heat a cast iron pan over medium-high heat. Add two tablespoons of olive oil and brown the tenderloin on all sides. Transfer to a wire rack over a lined baking sheet and roast in the oven for 35-40 minutes or until thermometer reads 130 degrees at center. Remove from oven to rest before slicing.
Heat a heavy-bottomed sauté pan over medium high. Add olive oil, farro, shallot and fennel. Sauté until fragrant (3-4 minutes). Deglaze the pan with sherry vinegar and red wine; let reduce by two-thirds. Add the chicken stock and aromatics and bring to a boil. Season with salt and fresh black pepper and cook until tender (30-35 minutes).Reserve.
For the shallots, preheat oven to 425 degrees. Film a cast iron pan with the olive oil over medium-high heat. Add the whole shallots to the oil and stir to coat evenly. Season with salt, a few turns of pepper, and the sprigs of thyme and place the pan in the oven. Roast the shallots until they are fully cooked through and easily pierced by a knife (approximately 45 minutes). Remove the shallots and allow cooling. Peel the shallots and arrange back in the pan. Drizzle with two teaspoons of the glaze and return to the oven for 5 minutes. Season and reserve.
To make the vinaigrette, combine vinegars, honey, mustard and truffle juice in a non-reactive bowl. Whisk to combine. While whisking, slowly incorporate the oils to emulsify. Season with salt and pepper to taste and reserve.
For the celery root discs, make an ice bath. Using a mandolin, slice celery root and quickly toss with lemon juice to prevent browning. Bring a seasoned pot of water to a rapid boil. Cook the disks until crisp-tender and shock in the ice bath. Pat dry and reserve in the vinaigrette.
To plate, spoon the farro onto six plates and top each with glazed shallots, marinated celery root discs and sliced veal. Finish with a drizzle of vinaigrette and garnish with tarragon sprigs.