Prep the spinach: Heat 1 teaspoon of olive oil in a skillet over medium. Add the minced garlic and cook for 20–30 seconds until fragrant. Toss in the spinach with a pinch of salt and pepper. Cook just until wilted, about 1–2 minutes. Squeeze a little lemon over it, then transfer to a plate and let it cool slightly.

Mash the avocado: In a small bowl, mash the avocado with a pinch of salt, pepper, and a few drops of lemon. You want it creamy but still a little chunky.

Prep the bread: Lightly butter one side of each bread slice. The buttered sides will face out for grilling. If using mustard, spread a thin layer on the unbuttered side of two slices.

Layer the fillings: On the unbuttered side of one slice, add a slice of cheese, a thick layer of mashed avocado, the warm spinach, and any extras like thin tomato or onion slices. Top with another slice of cheese. Finish with the second piece of bread, unbuttered side facing in, buttered side out.

Heat the pan: Wipe out the skillet and place it over medium to medium-low heat. Add a small pat of butter or a light drizzle of oil for extra crispness.

Grill slowly: Place the sandwich in the pan. Cook 3–4 minutes per side, pressing gently with a spatula, until the bread is golden and the cheese is melting. Adjust heat as needed so it browns without burning. Low-and-slow gives you the best melt and crunch.

Rest and slice: Transfer to a cutting board and let it rest for 1 minute. Slice in half with a sharp knife. Sprinkle with a pinch of flaky salt or red pepper flakes if you like a kick.