Pat the tuna dry. Use paper towels to remove excess moisture from both sides. Dry steaks sear better and form a nice crust.

Make the lemon garlic mixture. In a small bowl, combine olive oil, minced garlic, lemon zest, 1 tablespoon lemon juice, Dijon mustard (if using), salt, pepper, and red pepper flakes. Stir until smooth.

Coat the tuna. Brush the mixture over both sides of the steaks. You don’t need a long marinade—5–10 minutes is enough while you heat your pan or grill.

Preheat your pan or grill. For pan-searing, heat a heavy skillet (cast iron works best) over medium-high heat with a thin film of olive oil until shimmering. For grilling, preheat to medium-high and oil the grates.

Sear the first side. Place the tuna steaks in the hot pan. You should hear a clear sizzle. Don’t move them. Cook for 1½–2 minutes for rare to medium-rare, depending on thickness.

Flip and finish. Turn the steaks and cook another 1–2 minutes. Tuna is best when still pink in the center. If you prefer medium, add 30–60 seconds more per side. Avoid overcooking.

Rest briefly. Transfer to a plate and let rest for 2 minutes. This helps the juices settle.

Finish with freshness. Squeeze a little more lemon juice over the top and sprinkle with chopped parsley. Serve with lemon wedges.