Place 1 cup of the sugar in a large, heavy skillet, and cook over medium-low heat. When it melts and turns golden, add the pork, raise heat to medium, and stir to coat. The sugar will harden and become sticky, but as it cooks, it will melt again. Stir in the remaining sugar, salt, pepper, and fish sauce. Cover and cook 2 minutes. Stir in the garlic and sesame oil, and reduce heat to low. Simmer, uncovered, for 20 minutes to reduce the sauce. Fold in the sliced onion and cook 5-7 minutes, until translucent. The pork should be caramelized at this point. If not, raise the heat to medium, and sauté, stirring, until the sauce reduces more.

Transfer to a serving bowl and top with the sliced scallion. Serve hot, with steamed white rice.