Place ½ cup of the olive oil, arugula, basil, Parmesan, pine nuts, garlic, and lemon peel in bowl of food processor or blender. Process to a thick paste.
With the motor running, add remaining ¼ oil in a steady stream; blend until smooth. If pesto is too thick for your liking, add 1-2 Tablespoons of lukewarm water and process again.
To make sandwich, toast bread then spread with pesto. Top with tomato slices, cucumber slices, and avocado. Sprinkle with sea salt to taste, then black pepper or red pepper flakes to taste. Slice sandwich in half and serve immediately.
Place leftover pesto in a covered container and refrigerate for your next sandwich! It will keep for a few days. Stir before using.
Recipe by The Culinary Cellar at https://theculinarycellar.com/avocado-sandwich-with-arugula-pesto/