Begin by preparing your caramelized onions. Melt 3 tablespoons of butter, in a large skillet, over low heat. Add the onions and salt. Cook on low, stirring occasionally, until the onions soften and begin to caramelize, about 20-25 minutes. Sprinkle on the brown sugar and stir to combine. Cook for an additional 5-10 minutes. Remove from heat and set aside.

Preheat your oven to 350 degrees F.

In a small saucepan, melt the 1/2 cup unsalted butter over medium-low heat. Whisk in the mustard, honey, garlic powder and a dash of salt and black pepper. Cook for 1-2 minutes more and then remove from heat.

In a 9x13 inch baking dish, brush two tablespoons of the melted butter on the bottom of the dish. Take the bottoms of your rolls and place them in the baking dish. Layer on baby spinach, roast beef and brie cheese. Top with caramelized onions. Place the tops of the rolls on top. Brush the remaining melted butter mixture directly on top of the rolls.

Cover the dish with foil and bake for 15-20 minutes or until the cheese has melted.

Remove from heat and serve immediately. Enjoy!