Prepare the Tuna: Season the tuna steaks with garlic powder, onion powder, salt, and pepper.
Grill the Tuna: Heat grill to medium-high heat. Grill the tuna steaks for 2-3 minutes per side, or until cooked through. Remove from grill and let rest for 5 minutes before slicing.
Make the Creamy Garlic Sauce: In a small saucepan, melt butter over medium heat. Add garlic and cook for 1 minute, until fragrant. Stir in heavy cream, parsley, oregano, salt, and pepper. Cook for 1-2 minutes, or until the sauce is smooth and creamy.
Make the Salad: In a bowl, combine mixed greens, cherry tomatoes, and cucumber. Toss with olive oil, red wine vinegar, salt, and pepper.
Assemble: Divide the salad among plates. Top with sliced tuna steaks and drizzle generously with the creamy garlic sauce.