Preheat a grill or grill pan over medium-high heat. Brush the tuna steaks with olive oil and season with salt and pepper.
Grill the tuna steaks for about 2-3 minutes on each side for medium-rare, or until desired doneness. Remove from heat and let rest for a few minutes.
In a small bowl, combine the sour cream, mayonnaise, minced garlic, lemon juice, Dijon mustard, salt, and pepper. Mix until smooth and set aside.
In a large bowl, combine the mixed salad greens, cherry tomatoes, cucumber, red onion, and feta cheese. Drizzle with balsamic vinaigrette and toss gently to combine.
Slice the grilled tuna steaks and serve over the salad. Drizzle with creamy garlic sauce before serving.