Before starting, remove 2 tbsp butter (dbl for 4 ppl) from the fridge and set aside to come up to room temperature. Preheat the oven to 450˚F.Wash and dry all produce. Heat a medium pot over medium heat. While the pot heats, peel, then cut onion into 1/8-inch slices. When the pot is hot, add 1/2 tbsp butter (dbl for 4 ppl), then onions. (NOTE: Do not use the softened butter in this step or step 3; save it for step 4.) Cook, stirring occasionally, until golden-brown, 5-7 min. Reduce heat to medium-low, then add half the balsamic glaze and season with salt and pepper. Cook, stirring occasionally, until onions are dark golden-brown, 4-6 min. Remove the pot from heat.

Meanwhile, halve buns.Core, then cut apple into 1/4-inch slices. Cut tomato into 1/2-inch pieces. Cut brie into 1/4-inch slices.Add remaining balsamic glaze and 1/2 tbsp oil (dbl for 4 ppl) to a large bowl. Season with salt and pepper, then whisk to combine.

Heat a large non-stick pan over medium-high heat. When hot, add 1/2 tbsp butter (dbl for 4 ppl), then swirl the pan until melted. Add three-quarters of the apples, then sprinkle with 1/4 tsp sugar (dbl for 4 ppl). Cook, stirring occasionally, until warmed through, 1-2 min. Transfer apples to a plate to cool.

Meanwhile, spread 2 tbsp softened butter (dbl for 4 ppl) on cut sides of buns.Arrange bottom buns on a parchment-lined baking sheet, cut-side up. Set top buns aside.Toast bottom buns in the top of the oven until warmed, 3-5 min. (TIP: Keep an eye on buns so they don't burn!)

Spread Dijon on toasted bottom buns. Top with warm apples, caramelized onions, then brie. Arrange assembled bottom buns and buttered top buns on the same parchment-lined baking sheet, cut-side up. Toast in the top of the oven until brie is slightly melted, 3-5 min. (TIP: If you want meltier brie, continue toasting until desired doneness.)