Prepare the Avocado Spread: Mash the avocado in a bowl. Add a squeeze of lemon juice, a pinch of salt, and some freshly ground black pepper. Mix until smooth.
Assemble the Sandwich: Lightly toast the bread if desired. Spread a generous layer of the avocado mixture on one side of each slice of bread. Layer the turkey slices on one slice of bread. Add cucumber slices, tomato slices, fresh spinach or arugula and basil. Add red onion slices if using. Drizzle a little olive oil over the top.
Finish the Sandwich: Top with the other slice of bread, avocado spread side down. Press gently and cut the sandwich in half.
Mix the ingredients for lemonade. Serve over ice if desired.
Serve the sandwich with chips, pickle, and lemonade.