Wash mussels thoroughly and brush if necessary. Remove beards. Discard any open or damaged mussels. Drain in a colander.
Chop the garlic and roughly cut the onions. Chop parsley and reserve some leaves for garnish.
Heat olive oil in a large saucepan over medium-high heat. Add onion and garlic, sauté for 5 minutes until softened. Season with salt and pepper. Stir in tomato paste and chilli flakes and cook for 1 minute.
Add canned tomatoes, herbs de Provence, parsley, and white wine. Simmer for 3 minutes until slightly reduced.
Add mussels and stir to coat with sauce. Cover and cook over medium-high heat for 3 minutes.
Shake the pot firmly with oven mitts. Cover again and continue cooking for 3 more minutes (6 minutes total).
Add butter, cover, and shake the pot once butter melts (about 1 minute). Continue cooking covered for 4 more minutes (10 minutes total) until mussels are done.
Discard any mussels that remain closed. Divide among deep plates, garnish with reserved parsley, and serve with fresh baguette or ciabatta.
The Boss Kitchen editorial staff oversees content review, fact-checking, and recipe verification across the site. Published articles pass through the editorial team before going live, ensuring ingredient lists, techniques, cooking times, and nutritional claims hold up in a home kitchen. The team coordinates contributions across the site writers, handles reader corrections, and maintains consistency in measurement conventions, safety guidance, and dietary labeling. Posts under this byline typically represent team-reviewed reference material, site announcements, or editorial roundups rather than individual-author features, and they are held to the same sourcing standards as bylined recipe and product coverage.