Combine egg yolk, crab, chilli, garlic, and mustard in a bowl.
Slowly whisk in olive and vegetable oils until the mayonnaise emulsifies.
Season with salt, pepper, and lemon juice, then stir in buttermilk and coriander.
Toast seeds and nuts in a dry pan until fragrant and oils release.
Pound toasted mixture with olive oil and lemon juice into a rough paste.
Preheat the grill to high.
Drizzle sardines and lettuce with olive oil and season with salt, pepper, sumac, and chilli.
Grill lettuce for 2 minutes until charred and softened.
Grill sardines for 2 minutes on the skin side, then flip briefly to cook through.
Squeeze lemon juice over the fish and lettuce.
Serve sardines on a circle of crab mayonnaise, topped with dukkah and fennel sprigs.