Clean your mussels by removing the beard and dirt from the shells (either with a knife or scrubbing brush). If your mussels are cracked or open, they are dead, discard.
In a pot over low-medium heat, drizzle 4-5tbs of olive oil. Add onion, garlic, tomatoes and chilli flakes, sauté until onion is translucent and the tomatoes are soft.
Add mussels, white wine and butter into the pot. Cover and cook until the mussels have all opened up.
Add the juice and zest of a lemon and top with parsley. Serve with toasted bread.