Preheat the oven to 350°. In a pie plate, toast the walnuts for 10 minutes, or until well browned. Let cool, then coarsely chop.
Increase the oven temperature to 425°. In a bowl, toss the shiitake with 1/4 cup of the oil and season with salt and pepper. Spread the shiitake on the baking sheet and roast for about 25 minutes, or until tender and browned around the edges.
In a small bowl, cover the shallot with the sherry vinegar and let stand for 10 minutes. Add the thyme and whisk in 1/2 cup of the oil. Season with salt and pepper.
Heat 1 cup of the chicken stock in a microwave oven. Add the porcini and let stand for 10 minutes. Remove the porcini and coarsely chop; pour the soaking liquid into a bowl, stopping before you reach the grit.
In a saucepan, heat the remaining 2 tablespoons of oil. Add the onion and garlic and cook over moderate heat until translucent, 6 minutes. Add the farro, porcini and their soaking liquid, the remaining 5 cups of stock and the bay leaf. Bring to a simmer, then cover and cook over moderately low heat until the farro is tender, about 20 minutes. Drain well; discard the bay leaf.
Return the farro to the saucepan and toss with the walnuts, shiitake, sherry vinaigrette and parsley and season with salt and pepper. Transfer to a bowl and serve warm or at room temperature.
The recipe can be prepared through Step 5; refrigerate overnight.