Cook 1 pound chopped kale in salted boiling water until tender, 3 minutes. Drain. Add 2 tablespoons olive oil, 1 sliced red onion, 3 smashed garlic cloves and 1/4 teaspoon kosher salt to the pot. Cook over medium heat, stirring, until lightly browned, 10 minutes. Stir in 1/4 cup red wine vinegar and 1 tablespoon sugar. Add the kale and cook, tossing, 1 minute. Season to taste.