Samgyupsal sukju bokkeum (Stir-fried pork belly and bean sprouts)

Name: Samgyupsal sukju bokkeum (Stir-fried pork belly and bean sprouts)
Prep time: 5 min
Cook time: 10 min
Total time: 15 min
Cuisine: Asian, Korean
Ingredients: 8 ounces thinly sliced/shaved pork belly (대패 삼겹살) or other thinly sliced pork or beef (See note); 10 ounces bean sprouts (sukju, 숙주); 1/4 medium onion; 2 teaspoons minced garlic (or 2 -3 plump garlic cloves, thinly sliced); 2 or 3 scallions (or 2 ounces garlic chives (buchu, 부추)); 2 tablespoons soy sauce or oyster sauce (or 1 T each); 1 teaspoon sugar - optional; salt and pepper to taste