Slice steak against the grain into thin slices, about ½” thick. In a large bowl, whisk together marinade ingredients. Add steak and toss until coated. Marinate for 20 minutes or up to 4 hours.
In a large skillet (preferably cast iron) over medium-high heat, heat remaining 2 tablespoons oil. Shake off excess marinade from steak and season all over with salt and pepper. Add steak in a single layer, working in batches if necessary, and cook until seared on bottom, about 2 minutes. Flip steak, then add 3 tablespoons butter, garlic, lemon zest, lemon juice, and thyme. Baste steak in butter and cook steak to your liking, about 2 more minutes for medium. Remove steak and let rest on a plate. Pour out extra sauce and save for later.
Return clean skillet over medium heat and melt remaining 2 tablespoons lemon butter. Add spinach and season generously with salt and pepper. Cook, stirring often, until wilted, about 4 minutes.
Serve steak with spinach and extra butter sauce for dipping.
Recipe by The Best Video Recipes for All at https://recipes-for-all.com/lemon-butter-steak/