Add pearl barley and vegetable broth or water to a medium saucepan. Bring to a boil, then reduce heat to low.Cover and simmer for 25–30 minutes, or until the barley is tender but still chewy. Drain any excess liquid, fluff with a fork, and set aside to cool slightly.
Preheat oven to 425°F. Line a large baking sheet with parchment paper.Add broccoli, mushrooms, red bell pepper, red onion, and corn to the baking sheet. Drizzle with olive oil, then season with salt, pepper, garlic powder, and oregano. Toss until evenly coated.
Roast for 20–25 minutes, stirring halfway through, until the vegetables are tender and lightly caramelized around the edges.
In a small bowl or jar, whisk together olive oil, lemon juice, Dijon mustard, honey or maple syrup, garlic, salt, and pepper until smooth.
Add cooked barley and roasted vegetables to a large bowl. Pour the lemon vinaigrette over the top and toss until everything is evenly coated.Fold in feta cheese, parsley, basil or mint if using, and toasted nuts or seeds.
Serve the salad warm, at room temperature, or chilled. Add an extra squeeze of lemon juice or sprinkle of feta before serving if desired.