Caramelized Onion Bacon Grilled Cheese

Place the two yellow onions and blocks of gruyère and white cheddar on the acacia wood cutting board. Use the chef knife to trim off the onion root and stem ends, peel away the dry outer papery skin, then slice onions into uniform 1/8-inch thick half-moons, setting slices in a small pile on the board edge. Shred both cheeses using the large holes of the stainless steel box grater, collecting shredded cheese in the medium stainless steel mixing bowl, tossing gently to combine. Discard onion skins and cheese rinds in a small scrap pile at the board corner.

Place the 10-inch carbon steel skillet on the stovetop burner behind the cutting board over medium heat (350°F/175°C). Lay the 8 bacon strips in a single layer in the cold skillet, cooking for 8-10 minutes total, flipping halfway with the stainless steel tongs, until bacon is deep golden-red and crisp, with no remaining soft translucent white fat. Transfer cooked bacon to the matte cream ceramic plate lined with a single sheet of unbleached paper towel to drain excess grease, leaving 1 tablespoon of rendered bacon fat in the skillet for cooking onions.

Reduce stovetop heat to medium-low (300°F/150°C). Add 1 tablespoon of unsalted butter to the remaining bacon fat in the skillet, then add all sliced onions and 1/4 teaspoon of flaky sea salt. Cook slowly for 20-22 minutes, stirring occasionally with the beech wood spatula, until onions are deep golden brown, soft, and jammy, with no sharp raw crunchy bite. Stir in the fresh thyme leaves and freshly ground black pepper during the last 2 minutes of cooking to release their aroma. Transfer finished caramelized onions to the stainless steel mixing bowl (set aside after shredding cheese), then wipe the skillet lightly with a paper towel to prepare for grilling sandwiches.

Spread 1/2 tablespoon of softened unsalted butter evenly across one full side of each sourdough slice (8 slices total, buttered side facing out). Place 4 slices buttered-side down on the acacia wood cutting board, then layer evenly across each base slice: 1/4 of the mixed shredded cheese, 1/4 of the caramelized onions, 2 slices of crumbled crispy bacon, then a final 1/4 of the mixed cheese. Top each stack with a second bread slice, buttered-side facing up, pressing gently with the palm of your hand to adhere layers and prevent fillings from falling out during cooking.

Return the cleaned carbon steel skillet to medium-low heat (325°F/160°C). Place 2 assembled sandwiches in the skillet at a time (do not overcrowd the pan), cooking for 4-5 minutes per side, pressing gently with the beech wood spatula occasionally, until the bread is deep golden brown and crisp on the outside, and the cheese is fully melted and gooey in the center. Check the underside of each sandwich every 2 minutes to avoid burning, reducing heat slightly if the bread browns too quickly before the cheese fully melts.

Transfer cooked sandwiches to the matte cream ceramic plate (with paper towel removed), letting them rest for 2 minutes to allow the cheese to set slightly (this prevents all the cheese from oozing out when sliced). Cut each sandwich diagonally into two equal triangles, sprinkle with a tiny pinch of flaky sea salt if desired, then arrange for serving.