Start with the toppings. For pickled onion: Combine onion, vinegar, sugar, and 1/4 tsp. salt, stirring until sugar dissolves. Cover and let stand at room temperature 1 1/2 hours, stirring a few times. Drain before using. For slaw: Combine cabbage, lime juice, cilantro, mint and 1/4 teaspoon salt. For aioli: Stir together mayonnaise, 1 tsp. smoked paprika and lemon juice.