Brush mushroom caps until clean. Wash spinach, drain and cook in a dry pan over medium heat until wilted. Cool, squeeze out water and finely chop. Heat olive oil with butter in skillet over medium heat and cook onion until soft. Stir in walnuts, green onions and garlic and cook briefly while stirring. Add spinach and combine. Remove from heat and cool. Stir in feta cheese, salt, pepper and GREEN JALAPENO CHILLI SAUCE. Stuff mushroom caps with the mixture and sprinkle with parmesan cheese. Bake at 200°C for 8-10 minutes. Serve hot.
Filling can be made several hours ahead and stored in the refrigerator. Spoon into mushroom caps just before baking.
Fill 6 large mushroom caps as directed above to serve as a meal.