In a bowl or a small jar, combine the olive oil, balsamic vinegar, Dijon mustard, grainy mustard, garlic, salt, and pepper. Whisk or shake well until combined. You want the dressing to be emulsified and creamy. Set aside.
Pat the steak dry with paper towels and season it generously with kosher salt and black pepper. This step is crucial for getting a nice crust.
Heat a tablespoon of oil in a cast iron pan (or any heavy pan) over medium-high heat. Once hot, add the steak and cook for about three minutes. Look for a golden-brown crust forming on the bottom. This is where the flavor develops!
Flip the steak and cook for an additional 30 seconds. Add butter to the pan now, basting the steak for an extra layer of richness. Cook for about 90 seconds more, basting continuously, until the internal temperature reaches 120°F for a rare steak or your desired doneness.
Once cooked, allow the steak to rest for 5 to 8 minutes on a cutting board. This resting period allows the juices to redistribute, making every slice tender and juicy.
While the steak is resting, preheat the grill to medium-high heat. Brush the steak with oil and season it with salt and pepper. Grill the steak for about 3 to 4 minutes per side for a perfect sear, adjusting according to your preferred doneness.
After grilling, let the steak rest again for 5 to 8 minutes, then slice it thinly against the grain. This step is crucial for tenderness!
In a large bowl, combine the mixed greens, cherry tomatoes, cucumber, and red onion. Toss these ingredients to create a vibrant base for your Steak Salad.
Dress the salad with three-quarters of the dressing you prepared earlier, tossing gently to combine. This helps every bite pack a flavorful punch.
Transfer the salad to a serving platter and top with the sliced steak, avocado, and crumbled feta cheese. Drizzle with the remaining dressing and finish with a grind of freshly cracked black pepper. Serve immediately and enjoy the feast!