Before starting, preheat broiler to high. Wash and dry all produce. Core, then cut apple into matchsticks. Cut brie in half, crosswise and parallel to the cutting board. Halve buns. Whisk together vinegar, half the fig jam and 1 tbsp oil (dbl for 4ppl) in a large bowl. Add apples and toss to combine. Peel, then cut the onion into 1/4-inch slices.

Heat a large non-stick pan over medium heat. When hot, add 1 tbsp oil (dbl for 4ppl), then onions. Cook until slightly softened, 3-4 min. Add remaining fig jam and season with salt. Cook, stirring occasionally, until the onions are dark golden brown, 4-6 min. Carefully rinse and wipe pan clean.

Arrange bun halves on a baking sheet, cut side up. Spread mustard on bottom buns and top with brie halves, cut side up. Spread onion mixture onto top buns. Bake in the middle of the oven until brie melts, 3-5 min.

While sandwiches toast, carefully rinse and wipe pan clean. Heat over medium heat. Add the pepitas to the dry pan. Toast, stirring often, until golden-brown, 3-4 min. (TIP: Keep your eye on them so they don't burn!) Transfer to a plate.

Add arugula-spinach mixture and pepitas to the large bowl with apples and dressing. Toss to combine. Season with salt and pepper.

Top brie with top buns and cut sandwiches in half. Divide sandwiches and salad between plates.