- Place half the button mushrooms in a food processor; pulse 8 times or until finely chopped. Remove from processor. Repeat procedure with remaining button mushrooms. Add onion to processor; pulse 8 times or until finely chopped.
- Heat oil in a large nonstick skillet over medium heat. Add the onion; sauté 5 minutes. Add the button mushrooms; cook until mushrooms are tender and liquid evaporates (about 10 minutes). Stir in the oregano, salt, pepper, and nutmeg; cook for 2 minutes. Remove from heat. Add cheese; stir until cheese melts. Stir in parsley.
- Preheat oven to 200°C.
- Place one phyllo sheet on a large cutting board or work surface (cover remaining phyllo to prevent drying). Cut sheet in half lengthwise; lightly coat with cooking spray. Fold each phyllo piece in half lengthwise to form a (10cm-wide) strip.
- Spoon a level tablespoon of mushroom mixture onto 1 short end of each strip, leaving a 2cm border. Fold 1 corner of edge with 2cm border over mixture, forming a triangle; continue folding back and forth into a triangle to end of strip.
- Repeat procedure with remaining phyllo, cooking spray, and mushroom mixture. Place triangles, seam side down, on baking sheets coated with cooking spray.
- Lightly coat tops with cooking spray.
- Bake at 200°C for 20 minutes or until golden. Serve warm.