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.