Preheat oven to 500F.
Lightly grease a baking sheet with cooking spray and set aside.
Add vegetable broth to a soup pot and bring to a boil.
Add the mussels; cover with a lid and continue to cook over high heat for 4 minutes, or until they open.
Remove from heat.
Using a slotted spoon, transfer mussels to previously prepared baking sheet.
Discard the empty half of each shell.
Spoon a drizzle of the cooking broth over each shell and set aside.
Melt butter over medium-heat in a nonstick skillet.
Add garlic and cook for 3 minutes, or until lightly browned, stirring very frequently. DO NOT burn the garlic.
Stir in the rosemary, and continue to cook for a few more seconds.
Remove from heat.
Add panko crumbs, salt and pepper; stir until moistened and well combined.
Spoon prepared crumbs over the mussels.
Transfer to oven and bake for 4 minutes.
Remove from oven.
Garnish with chopped fresh parsley.
Serve.
