Steak Skewers with Scallion Dipping Sauce

Name: Steak Skewers with Scallion Dipping Sauce
Ingredients: 1 cup canned unsweetened coconut milk; 1 /4 cup fish sauce (such as nuoc nam or nam pla); 2 tablespoons palm sugar or (packed) light brown sugar; 1 tablespoon fresh lime juice; 6 garlic cloves; 4 red or green Thai chiles, stemmed; 2 lemongrass stalks, bottom third only, tough outer layers removed; 1 1 1/2' piece ginger, peeled; 2 pounds tri-tip, top sirloin cap steak, or rib eye, fat cap left on; 15 scallions, very thinly sliced; 3 tablespoons grapeseed oil; 2 tablespoons chinkiang (black) vinegar; 2 tablespoons toasted sesame seeds; 1 /2 cup canned unsweetened coconut milk; 3 tablespoons fish sauce (such as nuoc nam or nam pla); 1 1/2 tablespoons fresh lime juice; 2 garlic cloves, crushed