Shrimp dumplings (saeu mandu)

Name: Shrimp dumplings (saeu mandu)
Prep time: 30 min
Cook time: 10 min
Total time: 40 min
Cuisine: Korean
Ingredients: 32 dumpling wrappers (store-bought or homemade) (See note 1); 12 ounces (340 grams) raw shrimp (Peeled and deveined); 10 ounces (280 grams) green cabbage; 4 ounces (110 grams) fresh mushroom caps, stems removed ((shiitake, button, crimini, etc.)); 1/2 small onion; 1 teaspoon minced garlic; 1 teaspoon finely grated ginger (or juiced); 1 teaspoon soy sauce; 2 teaspoons sesame oil; salt to taste (about 1/4 teaspoon); pinch pepper; 1 tablespoon soy sauce; 1 teaspoon vinegar; 1 tablespoon water; pinch of black pepper (and/or gochugaru (red chili pepper flakes))