Heat oil in a medium nonstick skillet or grill pan over medium-high heat. Add the peppers and onions in a single layer. Cover and cook for 5 minutes.
Toss the peppers and onions, and add the mushrooms to the skillet. Place the sausage in with the vegetables and cook for 8-10 minutes, turning occasionally, until the vegetables are softened and the sausage is heated through.
Combine the mustard, hummus and yogurt in a small bowl. Top the pita with the sausage, mustard mixture, and vegetables. Season with pepper to taste.