Whisk garlic, balsamic vinegar, grill seasoning, oregano, salt, and pepper together in a large bowl. Add steak and toss to evenly coat. Cover bowl with plastic wrap and marinate in the refrigerator, 8 hours to overnight.
Heat olive oil in a skillet over medium-high heat until sizzling; add meat. Cook uncovered until liquids have drained from the steak, 3 to 5 minutes. Reduce heat to medium; cover.
Cook steak for 15 minutes; uncover and flip. Add water to the skillet as needed to keep steak moist. Continue cooking until slightly firm, hot, and lightly pink in the center, about 15 minutes. An instant-read thermometer inserted into the center should read 140 degrees F (60 degrees C). Transfer meat to serving plate; reserve liquid in the skillet.
Place onion in the skillet with reserved liquids; cook and stir until softened, about 5 minutes. Reduce heat to low; add mushrooms. Cook and stir until mushrooms are warmed through, about 5 minutes. Pour onion mixture over steak.
