Preheat the oven to 400 degrees F.
On a sheet pan, toss the tomatoes, corn and zucchini with 1 tablespoon olive oil, 1 teaspoon kosher salt and 1/2 teaspoon black pepper. Spread them into a single layer across the pan, arranging the tomatoes, so they are cut side up.
Roast the vegetables for 30-35 minutes until they are tender and just starting to brown at the edges.
While the vegetables are in the oven, make the barley. In a medium saucepan, combine the barley and water. Bring to a boil, reduce heat and simmer for 12 minutes until the grains are tender, but still chewy. Let stand for 2-3 minutes and fluff with a fork. Drain the remain water if necessary. Transfer to a large bowl.
For the vinaigrette, whisk the garlic, mustard, oregano, 1/2 teaspoon kosher salt, 1/4 teaspoon black pepper, vinegar, lemon juice and 1/4cup olive oil in a small bowl.
Stir half the vinaigrette into the barley.
Fold in the roasted vegetables, scallions and parsley. Drizzle in additional vinaigrette as desired.