Step 1: Prepare your ingredients. Slice your ripe avocado and tomato into even, thin rounds. If your mozzarella is a block, slice or shred it.
Step 2: Assemble your sandwich. On one slice of bread, lay out the avocado slices. Follow with the tomato slices, then top generously with the mozzarella. Season with a pinch of salt and pepper. Place the second slice of bread on top.
Step 3: Heat your pan. In a non-stick skillet or frying pan, melt the olive oil or butter over medium heat. Ensure the pan is evenly coated and hot but not smoking.
Step 4: Grill the first side. Carefully place the assembled sandwich into the hot skillet. Grill for 3-4 minutes, or until the bread is golden brown and the cheese begins to soften.
Step 5: Flip and finish. Using a spatula, gently flip the sandwich. Continue to cook for another 3-4 minutes on the second side, or until it’s golden brown and the mozzarella is completely melted and gooey.
Step 6: Serve immediately. Remove the sandwich from the pan, let it cool for just a minute, then slice in half. Garnish with fresh basil leaves if desired for an extra pop of flavor and color.