Drain the canned 3 can canned tuna in water drained well thoroughly, pressing out excess water with the back of a Dipping Fork or through a fine-mesh Colander. Transfer to a large Mixing Bowl and flake apart with a Dipping Fork.

Add the ¼ cup plain Greek yogurt full-fat or 2%, 2 tbsp mayonnaise, 1 tsp Dijon mustard, and 2 tbsp lemon juice freshly squeezed, about 1 lemon to the Mixing Bowl with the 3 can canned tuna in water drained well. Stir to combine. Fold in the diced 3 stalks celery finely diced and ¼ cup red onion finely diced. Season with ½ tsp fine sea salt and ¼ tsp freshly ground black pepper, taste, and adjust as needed. Stir in the chopped 2 tbsp fresh flat-leaf parsley finely chopped if using.

Separate the 1 whole head butter lettuce leaves separated and washed into individual cup-shaped leaves. Rinse under cold water and pat dry with Paper Towel. Arrange 8–12 leaves on a serving platter or individual plates.

Use the stem-flick test before cutting: flick off the small nub at the top. Green underneath means it's ready. Won't budge means it needs more time at room temperature. Missing nub or brown underneath means it may be overripe or bruised inside. Don't squeeze repeatedly — it causes bruising. If still firm, leave it on the counter away from direct sunlight until it yields gently to light pressure, which can take one to several days. Once ripe, rinse under cool water and place on a stable Cutting Board with a sharp Knife.