Preheat the grill to high heat.

Trim about an inch from the lettuce heart while making sure to keep it intact. Trim another inch from the top of the lettuce to remove some of the curly ends. Remove any wilted or browned leaves. Cut each lettuce head lengthwise in half.

In a small bowl, whisk together the olive oil, lemon juice, dijon mustard, garlic powder, salt and pepper. Brush the dressing over the lettuce on all sides, making sure to get the dressing between the leaves as much as possible.

Transfer the romaine lettuce to the preheated grill and grill on the flat side of the romaine lettuce until lightly browned without moving, about 2 minutes. Serve immediately whole or chopped with more lemon juice and freshly grated parmesan cheese, if desired.