Discard any mussels with broken shells and any that refuse to close when tapped. Place the mussels, thyme, bay leaf and wine in a saucepan and steam for 4–5 minutes, or until the mussels open. Discard any mussels that remain closed.
Remove the mussels from the pan with a slotted spoon and strain off the liquid through a fine sieve into a saucepan. Simmer this liquid for 10 minutes over a low heat, or until reduced by half. Pick the mussels and set aside.
Meanwhile, place the onion in a shallow dish and cover with the red wine vinegar. Leave to marinate for 10 minutes.
Add the broad beans to the mussel liquor, season with salt and pepper and stir in 1 tablespoon of olive oil.
Brush both sides of the bread slices with the remaining olive oil and toast under the grill. Divide the mussels between the bread slices and then spoon over the broad beans and marinated onions. Garnish with the sea vegetables and serve immediately.