This quick-cooking, whole-grain side is good with sauteed tofu, Italian sausage, or Garlic-Lemon Pork.
Thinly slice red onion and toss with vinegar; set aside. In a medium saucepan, bring water to a boil. Add barley. Reduce heat and simmer, covered, until barley is tender and water is absorbed, 10 to 12 minutes. Thinly slice kale leaves and add to a large bowl along with cooked barley, red onion mixture, tomatoes, cucumber, and bell pepper. Toss with olive oil and season with salt and pepper.