Place steak in a large bowl or a large ziploc bag.
In a separate mixing bowl combine olive oil, balsamic vinegar, lemon juice, worcestershire sauce, garlic, salt and pepper; reserve about ¼-cup of the marinade and set it aside for later to use as salad dressing.
Pour the rest of the marinade over the steak; cover and let marinate for at least 30 minutes on the counter, or up to 4 hours in the refrigerator.
In the meantime, work on the mushrooms: Set a cast iron skillet over medium-high heat, add butter and let it melt.
Stir in thyme and rosemary, then stir in the mushrooms; cook for 3 to 4 minutes, or until mushrooms are tender.
Remove mushrooms from skillet and set aside.
Return skillet to the burner and increase heat to high.
When skillet is hot, remove steak from marinade, shake off excess liquid, and transfer steak to the hot skillet; cook for 4 minutes, then turn it over and continue to cook for 4 to 6 more minutes, or until internal temperature of the steak registers at 125˚F to 135˚F.
Remove steak from skillet and transfer to a cutting board; let sit for 5 to 8 minutes before slicing the steak.
Meanwhile, arrange baby spinach leaves and shredded lettuce on a long serving platter or in a large salad bowl.
Add tomatoes and mushrooms to the salad.
Cut steak across the grain into ¼-inch thick slices.
Add slices of steak to the salad.
Top with red onions and reserved dressing/marinade.
Sprinkle with crumbled feta cheese and serve!