Prep the carrots and herbs. Grate the carrots on the large holes of a box grater. Finely chop the parsley and dill. Slice the spring onions or red onion thinly. Set everything aside.

Mix the dressing. In a bowl, whisk together Greek yogurt (or mayo), Dijon mustard, olive oil, the zest of half a lemon, and 1–2 teaspoons of lemon juice. Season with a pinch of salt and black pepper. Taste and adjust for brightness and creaminess.

Combine the salad. Add the grated carrot, herbs, and onions to the bowl. Toss gently to coat. If using capers, stir them in now for a briny pop.

Prep the mackerel. Drain the mackerel and flake it into large pieces. Remove any bones and skin if present. Squeeze a little lemon juice over the fish and add a crack of pepper.

Fold the mackerel into the salad. Add the mackerel to the carrot-herb mixture and fold gently so you keep some texture. You want a mix of small bits and larger flakes.

Taste and adjust. Add more lemon, mustard, salt, or pepper as needed. The flavor should be bright, savory, and balanced—not heavy.

Toast the bread (optional). Lightly toast your slices or warm your rolls. This adds crunch and helps the bread hold up to the filling.

Assemble. Layer lettuce or cucumber on the bread if using. Spoon on a generous amount of the mackerel salad. Top with a sprinkle of extra herbs or chili flakes if you like a little heat. Close the sandwich and press gently.

Slice and serve. Cut in halves or thirds. Serve with lemon wedges to squeeze over for extra brightness.