Wash and slit the green chillies carefully only on one side, remove seeds and keep aside. Peel and finely chop the onions. Wash the spinach thoroughly under running water. Remove stems, blanch in boiling water for a minute and take out immediately. Refresh in cold water, drain and chop the blanched spinach. Wash and chop the mushrooms.
Grate cheese and keep it aside. Preheat the oven. Heat oil in a pan on medium heat, add chopped onions and cook, stirring continuously, till onions turn light golden brown. Add the chopped mushrooms and cook for another two to three minutes, stirring frequently. Add the blanched spinach and cook for another two minutes.
Add cumin powder, red chilli powder and salt. Mix well. Remove from heat and cool. Add breadcrumbs and grated cheese to the mixture. Stuff the green chillies with the spinach and mushroom mixture and bake in a preheated oven at 200º C for about twenty minutes or until the chillies are softened. Serve hot.