Heat 1 teaspoon cumin in a small saucepan over medium heat until just toasted, about 3 minutes. Stir in vinegar and sugar. Bring to a boil; cook for 5 minutes. Add onion and reduce to a simmer. Simmer for 5 minutes. Let cool; cover and refrigerate for at least 4 hours.
Whisk olive oil, 1 teaspoon cumin, and 2 tablespoons juice from the pickled onions in a bowl. Mix in cabbage, cilantro, and half of the pickled onions. Stir in salt and black pepper. Refrigerate at least 4 hours.