Measure the barley into a strainer and rinse with cold water. Transfer the rinsed barley to a medium saucepan. Add the water and 1/2 tsp kosher salt. Cover and bring to a boil over high heat. Once boiling, reduce the heat to low and cook at a low simmer for 40 to 45 minutes until the barley is tender. Spread the barley on a parchment-lined baking sheet and allow to cool completely before frying.
Preheat the oven to 425°F. Wash the mushrooms and cut into quarters or thick slices if they are large. Transfer to a baking sheet and drizzle with olive oil. Season with the Wild Mushroom Seasoning Salt and toss to coat. Roast for 20 to 25 minutes until the mushrooms are tender and golden brown. Let cool for a few minutes until the mushrooms are warm, but not hot, before using in order to prevent the salad from wilting.
To fry the barley, heat 1 inch of vegetable oil in the bottom of a deep saucepan over medium heat. Line a baking sheet with paper towel and set aside. Drizzle the barley with a little oil, tossing to coat the individual grains so they don't stick together.
When the oil reaches 325°F it's ready to fry. You will need a fine mesh skimmer to remove the barley after frying. Carefully add about a third of the barley to the hot oil. Use a large spoon or the skimmer to add the barley to the oil to protect yourself from spatters.
Let the barley fry untouched for about 2 minutes before stirring. The barley will initially begin to stick together in clumps, this is normal. If you stir immediately after adding the barley it will only stick together more. After about 2 minutes, you can begin to stir the barley occasionally. As it browns, the barley clumps will begin to break apart. Fry for about 4 to 6 minutes until the barley is a deep brown colour.
Use a mesh skimmer to transfer the barley to the paper towel-lined baking sheet. Spread the barley grains out, breaking up any clumps with your hands. Season with kosher salt and let cool before using.
In a small bowl, combine the Dijon mustard, sherry wine vinegar, kosher salt, and pepper. Whisk together. Slowly add the light olive oil, whisking constantly. Keep whisking until the dressing comes together and is fully emulsified. Stir in the finely diced shallot.
To serve the salad, dress the arugula with some of the vinaigrette, tossing to coat. In a serving bowl, layer the arugula and roasted mushrooms with the crispy barley, tossing together lightly. Drizzle any remaining vinaigrette over the salad and top with a few roasted mushrooms and a sprinkle of the crispy barley. Enjoy!