To make the tunnbrod, pour the warmed milk and the melted, cooled butter into a bowl. Mix in the sugar, the dry yeast and the salt, and allow to stand for about 20 minutes.
In a separate bowl, mix together the strong white bread flour and the rye flour with the ground fennel seeds and the baking powder.
Next, either using a stand mixer with a paddle, or by hand using a wooden spoon, add the flour mixture a handful at a time to the milk, butter and yeast mixture, stirring all the time, until you have added everything and a rough dough has formed.
Now, either exchange the paddle for a dough hook and knead on medium high speed for about 7 minutes, until the dough is smooth and glossy. Or use your hands to knead the dough until it is smooth and glossy (about 10 minutes).
Transfer the dough to a bowl that has been lightly oiled with olive oil, then cover it and allow it to rise until it has doubled in size (about an hour in a warm place). Next, divide the dough into 12 roughly equal portions, then roll out each piece as thin as you can get it.
Heat a heavy (ideally, a cast iron) frying pan or griddle pan over a medium heat until it is quite hot. Prick each dough piece well with a fork, then put on the pan and cook for about 2-3 minutes on each side. Alternatively, you can bake them in the oven on baking trays at 200°C, 400°F, Gas 6, for about 6 minutes, turning them over halfway through.
Once the flatbreads are cooked, cover with tea towels and allow them to cool slightly.
To serve, spread each tunnbrod with the cream cheese, then add the gravlax or smoked salmon pieces, whichever you're using, followed by a sprinkling of the capers, the sliced radishes, the diced gherkins, the chopped chives and the dill sprigs.