Preheat grill to 350 degrees.

In a medium skillet, heat olive oil over a medium flame.

Add onion and cook until softened.

Stir in baby spinach and portobello mushroom slices. Cook until spinach is wilted and mushroom slices have softened.

Lower heat and pour in balsamic vinegar. Cook down until it's fully absorbed into the onion, spinach, and mushroom mixture. Season with salt and pepper.

Begin preparing each slice of bread with a tablespoon of mayo.

Spoon balsamic mixture onto one slice of bread.

Add a little brie to on top of each slice, then top each sandwich with the other slice of bread.

Grill sandwich for 2-3 minutes on each side, or until dark grill marks are achieved and brie is completely melted.

Serve grilled cheese immediately.