Saute the onions and peppers in a little oil until just soft. About 5 minutes (this step was completed the day before).
Butter one side of each piece of bread. Assemble the sandwich beginning with the smoked gouda on the first piece of bread. Next, add the shredded rump roast. Top with the sauteed peppers and onions. Next, put on the shredded mozzarella and top with the second piece of bread.
Heat a skillet on medium heat and add the sandwich to the pan. Cook about 3 minutes on each side. When flipped to the second side, put a cover on the pan to get a better melt on the cheese.
Serve and enjoy!