Pat the steak dry, then season generously with salt and pepper. Heat 1 tablespoon of olive oil in a cast-iron skillet over medium-high heat until shimmering.

Sear the steak in the hot skillet for 3-5 minutes per side for medium-rare, adjusting time for desired doneness; use a meat thermometer for accuracy.

Transfer the cooked steak to a cutting board, tent with foil, and let it rest for 5-10 minutes.

While the steak rests, whisk together balsamic vinegar, the remaining olive oil, Dijon mustard, and optional honey or maple syrup in a small bowl, then season with salt and pepper.

In a large bowl, combine the spinach, strawberries, avocado, and red onion (if using).

Thinly slice the rested steak against the grain.

Divide the spinach mixture among plates, top with sliced steak and feta cheese (if using), then drizzle generously with the balsamic vinaigrette and serve immediately.