Cut the lemon into slices and boil in a pot with 300 ml (approximately 1 cup) of water along with the fennel seeds. Rinse the mussels.
Pour the prepared seafood into the lemon water, cover and cook over medium heat for about 4 minutes, then remove from the broth with a slotted spoon. Place in a bowl and let cool. Remove the mussel meat from the shells.
Rinse and trim the scallions and cut into thin rings.
Rinse the lettuce hearts, shake dry and cut into thin strips.
Peel the grapefruit with a knife and cut the fillets from the membranes, saving the juice. Mix the lemon and grapefruit juice with 3 tablespoons of the cooking liquid and oil to make a salad dressing. Season the dressing with salt, pepper and a few splashes of pepper sauce.
Toss the scallions and strips of lettuce with the seafood and dressing in a large bowl.
Arrange the seafood salad on plates and distribute the grapefruit segments on top. Serve with fresh toast as desired.