In a mixing bowl, whisk together olive oil, soy sauce, honey, lemon juice, lemon zest, thyme leaves, and minced garlic. Place the flank steak in a resealable bag or shallow dish, pouring the marinade over it. Ensure the steak is fully coated, then seal the bag or cover the dish. Let it marinate at room temperature for at least 15 minutes.

Heat a large skillet over medium-high heat for about 2-3 minutes until hot. Remove the steak from the marinade, allowing the excess to drip off. Sear the flank steak in the skillet for 4-5 minutes on each side, or until it reaches your desired doneness.

After the steak is cooked, transfer it to a plate and cover it loosely with foil to rest. In the same skillet, reduce the heat to medium and add butter, letting it melt and bubble. Stir in the remaining marinade and a pinch of red pepper flakes.

Slice the rested flank steak against the grain into thin strips for tenderness. Arrange the slices on a platter and drizzle the rich lemon butter sauce generously over the top. Serve immediately on a bed of fresh baby spinach.