Preheat oven to 425 degrees Fahrenheit.
Bring 3 cups water to a boil then add barley and thyme. Reduce heat and simmer until barley is tender and liquid absorbed about 35-45 minutes.
Alternatively: Place water, barley, and thyme in Instant Pot. Close lid, set to sealing, and turn on to high pressure for 35 minutes. Conduct natural pressure release or quick valve. Drain and rinse when done.
While barley is cooking, place vegetables on a baking sheet. Drizzle with olive oil and season with salt and pepper to taste. Toss to coat and spread evenly on the sheet. Place in oven and roast for 20-30 minutes.
Combine all ingredients for the dressing, whisking until well combined.
When barley is done, drain excess water and rinse barley. Transfer to a large bowl, add roasted veggies and vinaigrette dressing. Mix until well combined. Serve immediately and enjoy!