This is a theoretical recipe. I made some mackerel pate and was eating it with raw fennel salad. I think it would be worth actually incorporating them into one dish, but I haven’t tried it yet.
The normal mackerel pate recipe is just smoked mackerel blended with enough creme fraiche to make a pate consistency, and lemon juice and parsley for flavour. Because it’s such a strong flavour, you could probably add really quite a lot of raw fennel. Quite coarsely chopped, I think, for a bit of crunch and a rustic quality.