01 -
Let your sandwiches sit for a minute or two. Cut them in half and dig in while they're still warm and melty.
02 -
Move the toast to a board. Spoon the onions onto two pieces, put the other bread slices on top, and give them a quick press to hold it all together.
03 -
Flip the bread. Throw the shredded Gruyère and cheddar over each slice. Pop a lid on and let it go till the bread looks golden and the cheese is completely melted—should take 2-3 minutes.
04 -
Heat up leftover butter and olive oil in your skillet on medium-low. Set all four bread slices down and let them toast till they get a little crisp, about 2 minutes.
05 -
Splash in your broth, sherry, or water, then scrape up all those browned bits with your wooden spoon. Stir in the thyme, let things cook for another minute, then pull the onions off the heat—put them in a heatproof bowl. Wipe your skillet clean.
06 -
With the lid off, toss in the sugar, salt, and pepper. Give the onions a good stir, put the lid back, then turn the heat down a notch. Keep an eye and stir every 5 minutes, cooking till the onions go dark golden brown—about 20 more minutes.
07 -
Start off by melting the butter and olive oil in a big, heavy pan over medium heat. Add in your sliced onions and stir so they're all coated. Cover up and let them soften for about 5 minutes.