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Mix stock, wine, tomato paste, thyme and garlic powder in medium bowl until well blended. Add mushrooms; toss to coat well. Set aside. Coat beef on both sides with pepper, pressing firmly so pepper adheres to the meat.
Melt butter in large nonstick skillet on medium-high heat. Add steaks; cook 4 to 5 minutes per side or until desired doneness. Remove steaks to plate; keep warm. Add onion to skillet; cook and stir 2 minutes or until softened. Add flour and sea salt; cook and stir 1 minute.
Pour mushroom mixture into skillet. Bring to boil and cook 10 to 12 minutes or until sauce is thickened and mushrooms are tender, stirring occasionally. Return steaks to skillet; cook 1 minute or until heated through. Season to taste, as desired.