1. Preheat the oven to 325 degrees . Season steaks with 1/4 teaspoon salt and 1/4 teaspoon pepper. In a small cup, combine miso and 2 teaspoons balsamic vinegar. Heat 1/2 teaspoon vegetable oil in a large skillet over medium-high heat. Brown steaks, about 2 minutes a side. Transfer steaks to a foil-lined baking sheet and roast 15 minutes for medium rare or until desired degree of doneness. Remove from oven and smear each steak with 1 teaspoon miso mixture. Set aside to rest. Stir 2 tablespoons cold water into remaining miso mixture.
2. Add 1 tablespoon vegetable oil to same skillet and heat over medium-high heat. Add 1 tablespoon minced garlic, kale, tomatoes, 1/2 teaspoon salt and 1/4 teaspoon pepper and saute 30 seconds. Add remaining miso mixture and saute 1 to 2 minutes, or until kale just starts to wilt. Remove from heat.
3. Thinly slice steak across the grain. In a large salad bowl, toss together kale mixture, steak and any juices that have accumulated, and onion.