Prep the Ingredients: Slice the avocado, cucumber, onion, tomato, and prepare the remaining sandwich ingredients.
Toast the Bread: Lightly toast the slices of sourdough bread.
Assemble the Sandwich: Spread mayonnaise on one side of each slice of bread. On one slice, layer arugula, spinach, tomato, turkey, provolone cheese, cucumber, onion, and pepperoncini. Place sliced avocado on top of the veggies.
Finish the Sandwich: Top with the second slice of bread, mayo-side down. Press gently to secure the sandwich, then slice in half.
Prepare the Lemonade: Mix lemon juice, water, and sugar to taste. Serve over ice if desired.
Serve: Serve the sandwich with homemade lemonade.