Heat oil in a frying pan over medium heat and then add the onions and sauté until soft, about 5 minutes.
Next, add the balsamic vinegar, sugar, and salt, stir to combine, and reduce the heat to medium-low to simmer, stirring occasionally.
While the onions cook, lightly toast the bread. Then add brie in an even layer to one side of each sandwich, then melt under the broiler (on low) for about 3 minutes or until it's nice and gooey.
After the onions cook for about 8 to 10 minutes, the balsamic should have thickened to coat the onions well. Remove them from heat and set aside.
Finally, assemble the sandwiches. Top the melted brie with sliced apples and balsamic onions, then close and serve.
Notes
Prep the brie - Our secret method to easily remove the brie rind is to scrape it off with a paring knife instead of cutting it off. It’s so much faster and means no wasted cheese! Check out our baked brie en croute recipe for more details!Toast the bread first - By toasting the bread before melting the brie, it won’t get soggy and will hold up with the perfect crunch in every bite.Slice the apple thin - Thinner apples allow for a better, more even coverage throughout the whole sandwich. Our goal is to get the perfect combination of ingredients in every bite. Caramelizing onions - Normally, caramelized onions take a much longer time, but we don’t want to do that for a simple lunch recipe. So we can cheat a little bit and speed up the process by cooking the onions over medium heat and adding a little salt and sugar. The salt helps to draw moisture out of the onions so they can caramelize faster, while the sugar itself caramelizes and adds the proper taste to the onion without waiting for them to caramelize themselves.