Cut the avocado in half, remove the pit, and scoop out into a bowl. Mash with a fork and add salt and pepper to taste.

Preheat your grill or sandwich press. While it heats, drizzle olive oil on one side of each bread slice.

On the non-oiled side of one slice, spread a generous layer of mashed avocado. Top with mozzarella slices and tomato. Optionally add basil or arugula before placing the other slice on top, oiled side up.

Carefully place the sandwich on the heated grill or sandwich press. Grill until the bread is crispy and golden, and the cheese is melting, flipping halfway if necessary.

Once done, remove the sandwich from the grill, let it rest for a minute, slice in half, and serve.