In a large skillet, over medium-high heat, heat the olive oil until shimmering.
Add in the ground beef and onions.
Cook 3-5 minutes, or until the beef is cooked through. Drain grease.
Mix in the ketchup, mustard, brown sugar, seasonings, and pickles. Remove from heat.
Heat a separate skillet over medium heat.
Spread about 1/2 tablespoon of softened butter on one side of each slice of bread.
To assemble, place one slice of bread face down on the skillet.
Add about 1/3 cup of cheese to the top of the bread. Spoon on a heaping 1/2 cup of sloppy joe mixture.
Sprinkle on an additional 1/3 cup of shredded cheese, and top with another slice of bread (buttered side up).
Cook for 1-2 minutes on each side.
Repeat with remaining ingredients and serve immediately.