Prepare the Marinade: In a small bowl or a mason jar, whisk together the balsamic vinegar, olive oil, Dijon mustard, minced garlic, honey, salt, and pepper. Reserve about one-third of this mixture to use as the final dressing for your Grilled Flank Steak Salad.

Marinate the Steak: Place the flank steak in a shallow dish or a large zip-top bag. Pour the remaining marinade over the meat, ensuring it is completely coated. Let the steak sit at room temperature for at least 30 minutes, or refrigerate it for up to 4 hours for a deeper flavor profile.

Preheat the Grill: Clean your grill grates and preheat the grill to medium-high heat.

Grill the Steak: Remove the steak from the marinade and discard the excess liquid. Place the steak on the hot grill. Cook for about 5 to 7 minutes per side, until the internal temperature reaches 130-135°F for medium-rare.

Rest the Meat: Transfer the steak to a cutting board and let it rest for at least 10 minutes.

Assemble the Salad: While the steak rests, toss the mixed greens, cherry tomatoes, and red onions in a large bowl with half of the reserved dressing.

Slice the Steak: Slice the rested flank steak very thinly against the grain.

Final Plating: Divide the dressed greens among four plates. Top each with the sliced steak, blue cheese crumbles, and toasted walnuts. Drizzle the remaining dressing over the top and serve immediately.