Mash the avocado and spread it on one slice of the bread.
Sprinkle with freshly ground black pepper, red pepper flakes (if using), and flaky sea salt.
On the other slice, spread a layer of pesto
To the mashed avocado slice, pile on the fillings: spinach leaves, fresh mozzarella, tomatillo, cucumber rounds, and a little pile of sprouts.
If you'd like to add a little protein, a poached or fried egg would be wonderful. My husband likes to add a few slices of bacon.
(Optional, but extra delicious: Once the sandwich is assembled, toast it in a panini press for a couple of minutes)