Prep the avocado. Scoop the flesh into a bowl. Add a squeeze of lemon or lime, a pinch of salt, and a few grinds of black pepper. Mash lightly with a fork until mostly smooth but still a bit chunky for texture.

Slice the cheese. Use enough to cover the bread in an even layer. Thin slices melt more evenly than thick blocks.

Butter the bread. Spread a thin, even layer of butter or mayo on one side of each slice. This is the side that will touch the pan for that golden crust.

Assemble the sandwich. Place one slice, buttered side down, on a cutting board. Add a layer of cheese, then a generous layer of avocado, then more cheese. Top with the second slice, buttered side up. The cheese-on-both-sides trick helps it melt around the avocado.

Heat the pan. Set a skillet over medium to medium-low heat. Give it a minute to warm up. Patience here prevents burning while the cheese melts.

Grill the first side. Place the sandwich in the skillet and cook until the bread is golden and the cheese starts to soften, about 3–4 minutes. If it’s browning too fast, lower the heat.

Flip and finish. Carefully flip with a spatula. Press down lightly to encourage contact. Cook another 2–4 minutes, until the second side is golden and the cheese is fully melty.

Add optional extras. If using delicate add-ins like fresh herbs or a handful of spinach, you can tuck them in after cooking by gently opening the sandwich. For tomatoes or bacon, add them during assembly so they warm through.

Rest, slice, and serve. Let the sandwich rest for 1 minute so the cheese sets slightly. Slice in half. Taste and add a pinch of flaky salt or a few red pepper flakes on the avocado side if you like.