In a medium sized bowl, mix balsamic vinegar, garlic, olive oil, and oregano.
Remove caps from mushrooms, place whole mushrooms, sliced peppers and onions into marinade bowl. Mix to distribute evenly. Cover and refrigerate for 30 min to 1 hour. (you can omit marinade time if time does not permit).
Heat a medium to large skillet over medium heat, add marinade and vegetables to cook for about 10 minutes. Add salt/pepper to taste. Stir onions and peppers a few times. Flip mushrooms once to cook both sides.
Place ciabatta bread into broiler for a few minutes, until the edges are slightly browned. Be very careful not to burn it!
Remove from oven. On the bottom slice, place 1 mushroom cap & half the onions/peppers. Top with feta cheese and arugula.
Spread 2 T hummus on top slice of bread and place on sandwich. Enjoy!