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.