Mediterranean Portobello Burger

Name: Mediterranean Portobello Burger
Ingredients: 1 clove garlic, minced; 2 cups loosely packed mixed baby salad greens; 1/4 cup crumbled reduced-fat feta cheese; 1/2 cup sliced jarred roasted red peppers; 1 tablespoon red-wine vinegar; 1/2 teaspoon kosher salt; 2 tablespoons extra-virgin olive oil, divided; 4 large slices country-style sourdough bread, cut in half; 2 tablespoons chopped pitted Kalamata olives; 1/2 cup chopped tomato; 4 portobello mushroom caps, stems and gills removed; 1/2 teaspoon dried oregano