In a large rectangular skillet, sauté red onion slices. While stirring, drizzle the onions with a little olive oil. Continue stirring until soft and caramelized. You can also add a tablespoon of water to help soften the onions. Remove onions from pan and set aside.
Spread the butter/olive oil on one side of each slice of bread. Flip the bread over and gently spread stone ground dijon mustard on 4 slices of bread and fig jam on the other 4 slices.
At this point, you can heat up the slices of ham in the skillet and lightly brown them. Then remove the ham and set it aside.
Start grilling the four slices of bread with butter and mustard in the pan, with the mustard side up.
Quickly place three slices of brie on the mustard side of half of the bread slices. Then place two slices of ham, followed by a scoop of caramelized onions. Place the jam side of the bread on top of the onions to complete your sandwich.
Cover the pan to help the cheese melt and the sandwich components to heat through.
After the first side is browned, use your spatula to carefully flip the sandwich and brown the other side. You may need to turn down the heat to finish melting your cheese without over browning your sandwich.