Preheat the oven to 400 degrees
Toss the zucchini and sweet potatoes in 3 Tbsp of the olive oil and season with the salt and pepper
Arrange the zucchini and sweet potatoes in a single layer on a baking sheet and bake for 20 minutes
Sautee the onion in the remaining olive oil until tender, about 7 minutes
Spread the outside of each piece of bread with 1 Tbsp mayonnaise
Spread the inside of each piece of bread with 1 Tbsp dijon mustard
Layer the sandwich with brie cheese, arugula, zucchini, onions, and sweet potatoes
Over medium heat, melt the butter in a cast iron skillet
Toast the sandwich on both sides until golden brown