Mediterranean Portobello Burger

1 clove garlic, minced
½ tsp kosher salt
2 tbsp extra-virgin olive oil, divided
4 portobello mushroom caps, stems and gills removed
4 large slices country-style sourdough bread, cut in half
½ cup sliced jarred roasted red peppers
½ cup chopped tomato
¼ cup crumbled reduced-fat feta cheese
2 tbsp chopped pitted Kalamata olives
1 tbsp red wine vinegar
½ tsp dried oregano
2 cups loosely-packed mixed baby salad greens

Preheat grill to medium-high. Mash garlic and salt on a cutting board with the side of a knife until it’s a smooth paste. Mix the paste with 1 tablespoon oil in a small dish. Lightly brush the oil mixture over portobellos and then on one side of each slice of bread. Combine red peppers, tomato, feta, olives, vinegar, oregano, and the remaining 1 tablespoon oil in a medium bowl. Grill the mushroom caps until tender, about 4 minutes per side; grill the bread until crisp, about 1 minute per side. Toss salad greens with the red pepper mixture. Place the grilled mushrooms top-side down on 4 half-slices of the bread. Top with the salad mixture and the remaining bread.