Pre-heat a non-stick pan to medium heat, or heat up your griddler (panini or sandwich maker/press).
On one slice of bread, spread a generous layer of prepared pesto . Place a generous layer of baby spinach leaves on top, followed by a layer of thick avocado slices.
Sprinkle some freshly ground black pepper and/or sea salt on top of the avocado layer.
Spread a generous layer of prepared pesto on the second slice of bread and sandwich on top.
Lightly butter both sides of the panini. Once the pan (or griddler/panini press ) is hot, grill the sandwich on both sides until golden brown, pressing down slightly with a spatula
Place the sandwich on a cutting board, cut in half, and enjoy as is or with a side of some balsamic vinegar or your favorite hot sauce for dipping.
Notes
I like using sour-dough bread slices for this sandwich, but you can use any bread you prefer. For gluten-free option, use your favorite gluten-free bread.Prepared Pesto - I typically have a batch of frozen home-made pesto in the freezer, making this sandwich quick to make. I use my Simple Pesto or Sun-Dried Tomato, Miso, Basil Pesto for for some added flavor (the Miso adds an element of "cheesy-ness" without the dairy of course), but you can just as easily sub any other type of pesto or store-bought pesto.