1. Place the romaine leaves on a large serving platter. Place the biggest spears at the bottom of the platter. Work toward the top using smaller and smaller spears to create a Christmas tree.
2. To a mixing bowl, toss to combine cherry tomatoes and mozzarella with pesto and olive oil. Mix well until tomatoes and mozzarella pearls are well coated in pesto.
3. Spoon Caprese mixture into each romaine leaf. Sprinkle with salt and pepper, if desired.