Preheat the grill 350 degrees F.
In a medium sized bowl, gently toss together the tomatoes, onion, peppers, feta, vinegar, cracked black pepper, to taste, and oregano. Set aside.
In a small bowl mix together the olive oil, garlic, and salt and pepper, to taste. Brush the portobello caps with the oil on and put them on the grill. Grill until tender, about 3 minutes on each side
Scoop a heaping tablespoon of the tomato mixture onto the bottom halves of the rolls, top each with some spring mix greens and a portobello cap. Cover with the top halves of the rolls and serve.