Rinse the pearl barley under cold water, then place it in a large pot with the water and 1/2 teaspoon salt. Bring to a boil, reduce heat to low, cover, and simmer for 25 minutes until tender. Drain any excess water and set the barley aside to cool.

Heat 2 tablespoons olive oil in a large skillet over medium heat. Add the sliced mushrooms and cook for 8 minutes, stirring occasionally, until they release their liquid and turn golden at the edges.

Stir in the minced garlic and cook for 1 minute more. Add the chopped kale and cook for 3 minutes until it softens but still holds some structure. Season with the black pepper.

In a small saucepan, combine the balsamic vinegar with the remaining 1 tablespoon olive oil. Simmer over low heat for 5 minutes until it thickens slightly into a reduction, then remove from heat.

In a large mixing bowl, combine the cooked barley with the mushroom and kale mixture. Drizzle the balsamic reduction over the top and toss gently to coat everything evenly.