If grilling, prepare grill ahead of time so it's hot and ready to go. If cooking in the oven, preheat temperature to 350 degrees.
To make the spinach pesto, add spinach, avocado, pecans, Parmesan, garlic clove, lemon juice, salt, pepper and water to the bowl of food processor. Process on high for 10-20 seconds, remove lid and scrap down sides, then process again until smooth consistency is reached.
To assemble the sandwiches, lather one side of bread with spinach pesto, add a few spinach leaves and Muenster cheese, then top with another piece of bread. Repeat to create the other three sandwiches.
Lay sandwiches on grilling pan to grill outdoors or baking sheet if cooking in the oven. Cook each side for approximately 3-5 minutes or until crisp bread is golden brown and cheese has melted.
*The Spinach Pesto serves approximately 8 - two tbsp per serving. *I only used about half of the Spinach Pesto to make these sandwiches, you can store the leftovers in the refrigerator for 3-5 days.