1. Combine soy sauce and sugar in medium bowl. Add beef, toss well, and marinate at least 30 minutes or up to 1 hour, tossing once.

2. Meanwhile, cover porcini mushrooms with 1/4 cup broth in small microwave-safe bowl; cover with plastic wrap, cut several steam vents in plastic with paring knife, and microwave on high power 30 seconds. Let stand until mushrooms soften, about 5 minutes. Lift mushrooms from liquid with fork and mince (you should have about 1 1/2 tablespoons). Strain liquid through fine-mesh strainer lined with paper towel into medium bowl. Set mushrooms and liquid aside.

3. Sprinkle meat with 1/2 teaspoon pepper. Heat 1 tablespoon oil in 12-inch skillet over medium-high heat until smoking. Add meat and cook until well browned on all sides, 6 to 8 minutes. Transfer to large plate and set aside.

4. Return skillet to medium-high heat and add remaining tablespoon oil, white mushrooms, minced porcinis, and 1/4 teaspoon salt; cook, stirring frequently, until all liquid has evaporated and mushrooms start to brown, 7 to 9 minutes. Scrape pan to loosen fond. Add onion and 1/4 teaspoon salt; continue to cook, stirring frequently, until onion begins to brown and dark bits form on pan bottom, 6 to 8 minutes longer. Add garlic, thyme, and flour; cook, stirring constantly, until vegetables are coated with flour, about 1 minute. Stir in remaining 1 1/2 cups beef broth and porcini soaking liquid, scraping bottom of pan with wooden spoon to loosen browned bits, and bring to boil.

5. Nestle steak pieces into mushroom and onion mixture and add any accumulated juices to skillet. Reduce heat to medium-low and simmer until steak registers 130°F on instant-read thermometer, 3 to 5 minutes, turning beef over several times. Season with salt and pepper, sprinkle with parsley, and serve.

Serves 4 to 6