Cook the barley: Cook the barley in the Instant Pot with chicken broth, and a little bit of salt, for 15 minutes on HI pressure. Quick release once the cooking time is up.
½ cup Pearl barley, 1 cup Chicken broth
Make the dressing: While the barley is cooking, mix together the dressing ingredients in a measuring cup or a mason jar. Because this dressing uses powdered mustard instead of liquid mustard, it may not emulsify completely - that is fine.
¼ cup Olive oil, ¼ cup Red wine vinegar, 1 teaspoon Garlic powder, 1 teaspoon Mustard powder, Salt and pepper to taste
Dress the barley: Once the pressure is released, transfer the barley to a large mixing bowl. While the barley is still hot, add the dressing to the barley and stir. Let the barley sit in the dressing for 5-10 minutes.
Finish the salad: Add the vegetables and feta to the barley, and combine. Serve room temperature or chilled.
½ English cucumber, 1 cup grape tomatoes, ¼ Red onion, ¼ cup Kalamata olives, 1 cup Greek feta cheese