Reserve 1/2 teaspoon salt; set aside. Place cabbage in a large bowl; sprinkle remaining salt over cabbage, sprinkling between leaves. Weigh down cabbage with a smaller bowl filled with cans. Let stand at room temperature for 2 hours; drain. Rinse cabbage thoroughly under cold water; drain. Remove cabbage leaves from core; discard core.
Combine reserved 1/2 teaspoon salt, onions, and remaining ingredients in a medium bowl. Spread radish mixture onto cabbage leaves; arrange leaves in layers in a 1-quart airtight container, pressing leaves to compress mixture. Top leaves with any remaining radish mixture. Cover and refrigerate 1 week. Store in an airtight container in refrigerator up to 2 weeks.