Prep Station → Slice mushrooms and onion paper-thin, mince garlic, strip thyme—set out cheese and butter.

Caramelize Onions → Heat olive oil in large skillet over medium-low; add onions with pinch salt—cook 8 minutes until golden, stirring occasionally.

Mushroom Magic → Push onions aside; increase heat to high, add mushrooms in one layer—dry-sauté 3 minutes until liquid evaporates.

Flavor Boost → Stir in garlic and thyme; cook 1 minute—deglaze with balsamic splash if using.

Butter Bread → Spread thin butter layer on one side of each bread slice—this becomes the crispy exterior.

Assemble Melt → On unbuttered side: layer Swiss, mushroom-onion mix, more Swiss—top with second slice, buttered side out.

Press & Cook → Heat skillet medium; add sandwich—press with spatula or heavy pan—cook 3 minutes until golden.

Flip Carefully → Turn sandwich; press again—cook 3 minutes until cheese melts and bread crisps.

Cover Trick → If cheese lags, cover 30 seconds—melts perfectly without soggy bread.

Rest & Serve → Rest 2 minutes on board; slice diagonally—admire the cheesy pull and dig in hot.