Rinse the mussels in cold water, scrub off any grit and pull off the hairy beards. Tap any open mussels on the counter: discard any that do not close within a minute, and any with cracked shells.
Finely dice the onion and slice the garlic.
Heat the oil in a large wide pot with a lid over medium heat. Add the onion and garlic and cook, stirring, for 3 minutes until soft.
Add the diced tomatoes, broth and chili flakes and simmer for 5 minutes.
Turn the heat to high, add the mussels, cover tightly and steam for 4 to 5 minutes, shaking the pot once, until the shells open.
Discard any mussels that stay closed. Stir in the parsley and lemon juice.
Ladle the mussels and broth into bowls and serve with crusty bread for the broth. Put out an empty bowl for the shells.