Vietnamese Cabbage Salad with Shrimp (Gỏi Tôm)

Ingredients

• 2 Thai chilis ((or 1 serrano chili; thinly sliced))
• 2 tablespoons fish sauce
• 3 tablespoons fresh lime juice
• 1 tablespoon minced fresh ginger
• ½ teaspoon kosher salt
• 3 tablespoons sugar
• 3 tablespoons water
• 1 onion ((halved and thinly sliced))
• 4 cups shredded green cabbage ((preferably napa cabbage))
• 1 carrot ((shredded))
• ½ seedless cucumber ((thinly sliced))
• ½ pound cooked shrimp ((halved lengthwise))
• 1/4 cup cilantro leaves ((Vietnamese coriander if possible))
• ¼ cup mint leaves
• 1/4 cup Thai basil leaves ((optional))
• Fried shallots ((to serve; optional))
• ¼ cup crushed roasted peanuts ((to serve; optional))