I always use frozen sweetcorn so I boil this first in some salt water.
While your waiting for that, chop the remaining ingredients as small as possible.
drain the Tuna if your using another kind, but keep the oil if its olive oil, sprinkle it apart with your fingers on top of your chopped veggies. (So that they are slightly smaller chunks but not flakes.)
Add the sweetcorn once cooled, followed by the ready mixed mayo, lemon juice and spices.
I like to use my hands to mix it together as I find it stays a bit more upright, you don't want it too mushy, just enough for the ingredients to be mixed and stick together ;)
Now the filling is sorted, place in the fridge to cool while you prepare the bread shapes
Choose your prefered cookie cutter shape, I used a round one. Then cut out and butter as many as you think you might need.
Then place a tea spoon full of filling on each one then gently pat down
Sprinkle a little mustard seeds on the top and garnish with a sprig of fresh dill