Grilled Portobello Burger

Name: Grilled Portobello Burger
Ingredients: 7 plum tomatoes, seeded and diced; 1/2 cup diced red onion; 2 red peppers, grilled, seeds removed; 1 cup crumbled feta cheese; 1 tablespoon red wine vinegar; 1 tablespoon cracked black pepper, or to taste; 1 1/2 tablespoons freshly chopped oregano leaves; 3 tablespoons extra-virgin olive oil; 2 tablespoons minced garlic; 1 tablespoon salt, or to taste; 1 tablespoon pepper, or to taste; 7 portobello mushroom caps, stems and gills removed; 7 sourdough rolls, split in 1/2; 2 cups spring mix greens