Before starting, remove 2 tbsp (4 tbsp) butter 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 (1 tbsp) butter, then onions. (NOTE: Do not use 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. 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 (1 tbsp) oil 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 (1 tbsp) butter, then swirl the pan until melted. Add three-quarters of the apples, then sprinkle with 1/4 tsp (1/2 tsp) sugar. Cook, stirring occasionally, until warmed through, 1-2 min. Transfer apples to a plate to cool.

Meanwhile, spread 2 tbsp (4 tbsp) softened butter 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 and 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.)

Meanwhile, add spinach, tomatoes and remaining apples to the bowl with vinaigrette. Toss to combine. When brie is slightly melted, spoon fig spread over top buns, then close melts. Halve crosswise, if desired. Divide melts and salad between plates. Sprinkle salad topping mix over salad.