Boil stock with onion, garlic, carrot, and celery.
Add haricot beans and cook for 30-40 minutes until tender.
Clean mussel and cockle shells, discarding any open ones.
Sauté shallots, garlic, and thyme in butter.
Add shellfish, prawn heads, tails, and wine to steam closed until open.
Pick meat from mussels and cockles and reserve the cooking liquid.
Preheat the grill to high or the oven to 220°C.
Cook fennel in oil, then add tomatoes, beans, and reserved stock.
Add prawns, scallops, and reserved shellfish meat to the pan.
Stir in lemon thyme, parsley, lemon zest, and juice.
Spoon mixture into shells, top with breadcrumbs, and grill until golden.