Brush mussels thoroughly under cold running water. Discard any open mussels.
Finely chop 1 onion and 3 cloves garlic. Sauté in 3 tablespoons olive oil over medium heat until translucent (about 2-3 minutes).
Pour in 300 ml white wine and bring to a boil over high heat.
Add mussels, reduce heat to medium, cover, and cook until all mussels have opened (about 5-8 minutes). Discard any mussels that remain closed.
Remove mussels from the broth and drain well. Set aside in a warm place.
Strain the broth through a clean kitchen towel.
Finely chop the remaining onion and 3 cloves garlic. Sauté in the remaining 3 tablespoons olive oil over medium heat until translucent (about 2-3 minutes).
Add breadcrumbs and cook for 1-2 minutes over medium heat, stirring occasionally, until lightly golden.
Pour in the remaining 100 ml white wine and the strained broth.
Add bay leaves, salt, and pepper. Simmer over low heat for 6-8 minutes.
Season with lemon juice to taste (about 3 tablespoons).
Return mussels to the sauce and stir.
Wash parsley, chop finely, and stir into the sauce. Heat for 1-2 minutes until warmed through.