Nutty pearl barley stands in seamlessly for rice in this hearty risotto starring garlicky sauteed mushrooms and a generous dose of fresh dill.
Heat 2 tablespoons oil in a large straight-sided skillet over medium-high. Add mushrooms; season with salt. Cook until browned in places, 6 to 8 minutes. Stir in garlic; cook 1 minute more. Transfer to a plate; reserve skillet for risotto (do not wipe skillet clean).
In a saucepan, combine broth, 2 cups water, lemon zest, 1 1/4 teaspoons salt, and 1/4 teaspoon pepper. Bring to a simmer over medium heat. Meanwhile, heat reserved skillet over medium. Swirl in remaining 2 tablespoons oil. Add leeks, season with 1/4 teaspoon salt, and cook, stirring occasionally, until soft and translucent, about 5 minutes. Add barley and cook, stirring, until translucent and grains begin to make a popping sound, 3 to 4 minutes. Stir in wine; cook until mostly evaporated, about 30 seconds.
Ladle 1 cup broth mixture into skillet. Stir with a wooden spoon at a moderate speed until most of broth is absorbed and bottom of skillet stays visible for a second or two as you mix, 2 to 3 minutes. Continue ladling broth into skillet, about 1/2 cup at a time (or enough to just cover barley). Stir occasionally, allowing barley to absorb most of broth before adding more. Risotto is ready when barley is tender and creamy and broth has thickened to a rich sauce, 1 hour. (You may not need to use all of broth.)
Remove from heat. Fold in mushroom mixture, butter, lemon juice, and cheese; season with salt and pepper. Top with dill and more cheese; drizzle with oil and sprinkle with more pepper to serve. If the risotto becomes stiff before serving, stir in more hot broth, a little at a time, until it's loose and creamy.
Covering barley with water, soaking at room temperature for 8 to 12 hours, and draining will reduce the grain's cooking time by half.