In a small bowl whisk together the red wine vinegar, garlic, mustard, salt, and pepper.
Slowly drizzle in the olive oil and whisk until a smooth, emulsified dressing is formed.
Lightly brush olive oil on both sides of the cut romaine lettuce.
Sprinkle salt and pepper on both sides.
Preheat grill over medium-high heat, about 350 to 400ºF (177 to 204ºC).
Clean the grill and then carefully grease the cooking grates with tongs using a paper towel dipped in oil.
Once the grates are hot, place the Romaine and lemon halves cut side down on the grill. Press the lettuce down so it creates direct contact with grates.
Cook until lettuce is lightly charred, 2 to 3 minutes.
Flip and cook 1 to 2 minutes until slightly wilted.
Transfer lettuce and lemon halves to a serving plate.
Sprinkle the parmesan cheese on top of the grilled lettuce.
Drizzle the dressing on the grilled romaine and squeeze the lemon on top.
Notes
Larger romaine hearts can also be cut into quarters.
Make sure to sear each cut area for 1 to 2 minutes. If the sections are slightly wilted and charred, do not flip over. Grill the uncut side since the sections are smaller.