Prep the bread: Lay out your slices of sourdough bread. Spread a thin layer of butter on one side of each slice—this will be the side that crisps up in the skillet.

Assemble the inside: On the unbuttered side of two slices, spread 1–2 tablespoons of pesto each. Make sure you get into the corners for that full-flavor bite. Sprinkle mozzarella and cheddar generously over the pesto.

Close the sandwich: Top with the remaining slices of sourdough, butter side facing out.

Grill it to perfection: Heat a nonstick skillet or cast-iron pan over medium heat. Carefully place your sandwiches in the pan. Grill for 3–4 minutes on each side, pressing gently with a spatula, until the bread is golden brown and the cheese has completely melted.

Serve warm: Slice in half and serve immediately. These are especially good dipped into tomato soup or served with a drizzle of balsamic glaze.