Cook spinach or broccoli according to package directions; drain well, pressing out the excess liquid. Set aside.
Rinse fresh blue mussels in tap water. Add water to a large covered pot. Add the mussels and cover. Steam 5 to 7 minutes
Drain mussels, reserving liquid. Add milk to mussel liquid to equal ½ cup. If desired, set aside 8 mussels for garnish.
In bowl combine eggs, milk mixture, spinach (or broccoli), cheese, ¼ cup of bread crumbs, onion, and mussels. Spoon mixture into custard cups or pastry shells. Place in shallow baking dish.
Combine remaining bread crumbs, parmesan cheese, and butter; sprinkle over mixture in cups. Bake at 350oF for 20 minutes. Garnish with the reserved mussels.