Bring dried porcini and 8 1/2 cups water to a boil over high heat, covered. Remove from heat and let sit 1 hour. Lift mushrooms from soaking liquid and chop coarsely; set liquid and mushrooms aside.
Meanwhile, melt butter with oil in a large cast-iron skillet over medium heat. Add onion, garlic, and farro and cook, stirring, until onion is translucent and farro turns toasty brown, about 7 minutes. Stir in 1 tsp. salt.
Strain porcini soaking liquid through a cheesecloth-lined strainer into a bowl. Pour 1 cup liquid into farro and cook, stirring, until nearly absorbed, about 5 minutes. Repeat until all liquid has been used and farro is tender, about 1 hour.
Stir in thyme, parsley, reserved porcini, and cheese, and cook another few minutes to meld flavors. Season with salt to taste.
*Find semipearled farro at amazon.com, or use regular farro: Soak 1 hour in 2 in. of water to cover, then drain and cook as directed--except add all the liquid at once (no amount of stirring will make it creamy, so save yourself the trouble).