Heat olive oil in a large frypan over medium-high heat. Saute bean sprouts, spring onion, mushrooms and olives until tender, about 3 minutes. Reduce heat to low and add the carrots, celery, ginger, salt and pepper. Cook for a few more minutes to blend the flavours. The carrots and celery should remain crisp. Stir in the black beans and sunflower seeds and cook just until heated through. Remove from heat.
Meanwhile, place some of the grated cheese inside each pita quarter and heat in the microwave for about 30 seconds, or until cheese has melted. Set aside.
Spoon vegetable mixture into the pita triangles. Garnish with sour cream and chives.