1. Combine sugar and molasses in a small heavy saucepan over medium heat. Cook 3 minutes or until sugar melts, stirring occasionally. Stir in hoisin, vinegar, 1 tablespoon soy sauce, and garlic; bring to a boil. Cook 5 minutes or until slightly thickened, stirring frequently. Remove from heat; stir in mustard. Keep warm.
2. Heat a grill pan over medium-high heat. Coat pan with cooking spray. Sprinkle 1/4 teaspoon salt and 1/4 teaspoon black pepper over pork. Add pork to pan; cook for 2 minutes on each side or until done. Add pork to hoisin mixture, turning to coat. Keep warm.
3. Add onions to pan; cook 2 minutes or until wilted. Remove onions from pan; chop. Combine onions, the remaining 1/4 teaspoon salt, and rice; serve with pork.