1. Combine onion, vinegar, sugar, and a pinch of salt in a medium bowl. Let sit 15 minutes, stirring occasionally.
2. Meanwhile, pat steak dry with paper towels and season with salt and pepper. Heat oil in a medium skillet over medium-high heat until just smoking. Add steak and cook until thickest part registers 125 degrees for medium-rare, about 5 minutes per side. Transfer to a cutting board and let rest 10 minutes.
3. Place feta in a small bowl and mash with a fork until no large pieces remain. Stir in yogurt, tahini, and garlic; season with salt and pepper to taste and set aside.
4. Microwave naan pieces until warm, about 10 seconds. Thinly slice steak across the grain and divide over naans. Top naans evenly with spinach and onions, then drizzle with feta sauce. Serve with cucumber slices.
Source: Hannaford fresh Magazine, October - November 2018