Melt butter in a Dutch oven and saute onion and mushrooms until all liquid is gone. The final mixture should not be runny. Add shellfish and fish and stir. Remove from heat and add parsley, ricotta, eggs, salt, and pepper and mix well. Preheat oven to 400°.
Butter a large baking sheet or use a nonstick one. Place 2 sheets of phyllo pastry on sheet and cover the rest with a damp cloth to prevent drying. Brush the pastry lightly with melted butter out to edges and repeat with remaining sheets, brushing every second one with butter.
Spoon seafood mixture in a long even shape down center of pastry; carefully fold pastry and roll into a strudel. Tuck ends under and brush with butter. Reduce oven heat to 375° and bake strudel 30 minutes, until top is browned. Brush top with butter twice during baking. Cool slightly, slice, and serve.