Preheat your grill or stovetop grill pan over medium-high heat.
Rub the tuna steaks with olive oil, then season with salt, pepper, and lemon juice.
Grill the tuna steaks for about 2-3 minutes on each side for medium-rare, or longer if you prefer them more cooked. Remove from heat and let rest for a few minutes.
In a small bowl, combine sour cream, mayonnaise, minced garlic, dill, lemon zest, and season with salt and pepper. Mix until smooth.
In a large bowl, toss together the mixed greens, cherry tomatoes, cucumber, red onion, and feta cheese. Drizzle with balsamic vinaigrette and toss to combine.
Slice the grilled tuna steaks and serve on a plate alongside the salad. Drizzle the creamy garlic sauce over the tuna or serve it on the side.