Heat olive oil in a large skillet over medium-high heat.
Season the tuna steaks with salt and pepper.
Once the oil is hot, add the tuna steaks and sear for about 2-3 minutes on each side for medium-rare, or longer if desired.
Remove from heat and let rest.
In a bowl, combine the sour cream, mayonnaise, minced garlic, lemon juice, and Dijon mustard. Mix well and season with salt and pepper to taste.
In a large salad bowl, combine the cherry tomatoes, cucumber, and mixed greens. Toss gently to combine.
To serve, slice the tuna steaks and place them on a plate alongside the salad.
Drizzle the creamy garlic sauce over the tuna and garnish with chopped parsley.
Serve with avocado slices on the side.