How to Make It
Spread the bottom half of the loaf with the pesto. Top with the turkey, cheese, and onion. Sandwich with the top half of the loaf and cut into 4 pieces. Dividing evenly, brush the outside of the sandwiches with 1 tablespoon of the oil.
Heat a grill pan over medium heat and cook the sandwiches, pressing down occasionally, until the bread is crisp and the cheese has melted, 5 to 6 minutes per side.
Meanwhile, in a large bowl, whisk together the vinegar, the remaining 2 tablespoons of oil, and ¼ teaspoon each salt and pepper. Add the lettuce and avocado and toss to combine. Serve with the sandwiches.