1. Wash spinach and arugula in plenty of cold water to remove grit. Remove thick stems. Put into a large pot with the water still clinging to the leaves. Steam until wilted, about 3–5 minutes. Drain and cool. Squeeze out excess moisture and chop fine.
2. Remove the stems from the mushroom caps. Place the caps, stem side up in a lightly oiled baking dish.
3. In a large non-stick frying pan, sauté the onions in oil until soft. Finely chop the mushroom stems and adds to the pan with the onions. Sauté for 3 minutes.
4. Remove form the heat and stir in the spinach/arugula mixture, cottage cheese, parmesan, sill, soy sauce and pepper.
5. Divide the mixture among the mushroom caps, mounding it. Bake at 400°F for 20 minutes or until lightly browned.