Place steaks and soy sauce in large resealable plastic bag. Seal bag, releasing excess air; turn to coat. Let steaks marinate at room temperature 2 hours, turning occasionally.
Process porcini mushrooms in spice mill to fine powder. Mix 1 1/2 tablespoons mushroom powder, rosemary, and pepper in small bowl (reserve any remaining powder for another use).
Drain steaks; pat dry. Sprinkle mushroom-rosemary rub generously over both sides of steaks, pressing to adhere.
Heat oil in heavy large skillet (preferably cast-iron) over medium-high heat. Fry steaks until browned and cooked to desired doneness, about 8 minutes per side for rare, adjusting heat to medium if browning too quickly.
Transfer steaks to plate; tent with foil to keep warm. Let rest 10 minutes. Cut into 1/2-inch-thick slices and serve.
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