1- Cook onions in a large frying pan till soft- about 5 minutes2- Slowly add spinach until it is all wilted and cooked down3- Remove spinach and onions to a large mixing bowl4- Add all remaining ingredients except mushrooms and gruyere and mix well- add salt and pepper to taste5- Stuff filling into mushrooms and sprinkle with gruyere6- Bake at 350 for about 35 minutes depending on size of mushroom