In a skillet or saucepan, heat the oil over medium-low heat. Once hot, add in the sliced onion and cook, stirring occasionally, for 50-60 minutes. Halfway through cooking, add a pinch of sugar and a dash of salt. If the onions begin to burn at all, turn the heat down lower. The idea here is low and slow.
When the onions are nearly done, prepare the bread. Butter one side of each piece of bread and then place the pieces butter side down. Top 2 slices with 1/3 cup of the cheese each (leaving 1/3 of the cheese remaining).
Once the onions are done, divided them evenly on top of the two pieces of cheesy bread. Divide the remaining cheese evenly among the 2 sandwiches. Add black pepper to taste and then top with the other slices of bread, butter side up.
Lightly clean out the skillet used for the onions and put over medium-low heat. Place the sandwiches in the skillet and cook until golden brown on each side, about 4-5 minutes per side. ENJOY!