Saute the onions until softened, then add the mushrooms and continue cooking until the mushrooms release their liquid. Drain the liquid and remove the onions and mushrooms to a mixing bowl. Mix in the cheeses, eggs, spinach and dill, then season to taste.
Spread the mushroom-spinach mixture into a greased baking dish, then place the other pastry sheet on top.
Bake at 350 degrees until the top is golden brown, about 15 to 20 minutes.