Combine 3 cups boiling water and dried porcini mushrooms in a bowl; cover and let stand 30 minutes. Drain through a sieve over a bowl, reserving the soaking liquid. Finely chop the mushrooms.
Preheat oven to 350°.
Heat 1 tablespoon oil in a large saucepan over medium-high heat; swirl to coat. Add onion; sauté 2 minutes, stirring frequently. Reduce heat to low; cook 30 minutes or until onion is tender and lightly browned, stirring occasionally. Add reserved porcini liquid, chopped porcini, farro, and 1/2 teaspoon salt; cover. Bring to a boil; reduce heat, and simmer 30 minutes or until farro is al dente and liquid is reduced to about 1/3 cup. Remove from heat.
Heat remaining 1 tablespoon oil in a large skillet over medium-high heat; swirl to coat. Add shiitake mushrooms, celery, thyme, and sage; sprinkle with remaining 1/2 teaspoon salt and pepper. Sauté 6 minutes or until mushrooms are lightly browned, stirring occasionally. Add wine to skillet; cook 3 minutes or until liquid evaporates. Add shiitake mixture to farro mixture; stir to combine. Spoon stuffing into an 11 x 7-inch glass or ceramic baking dish coated with cooking spray; cover dish with foil. Bake at 350° for 30 minutes. Let stand for 5 minutes, and top with celery leaves.
Made this for Thanksgiving and it was a huge hit!!! There is already talk of repeating it at Christmas dinner. It offered an excellent vegetarian option, and the mushrooms make it more substantial than just a simple side dish. I would however say that calling it 'stuffing' is a misnomer. This is a superb dish, but if you are looking for a true stuffing this dish will fall short
Served this instead of traditional bread stuffing for Thanksgiving. It was great, very earthy and nice to have a vegetarian option. Subbed wheat berries for the farro (couldn't find it at a reasonable price), they were nice and chewy.