Preheat oven to 400F/200C.
Boil the potatoes until cooked - no need to peel before cooking nor cut. Drain and allow them to cool once done (this can be done ahead of time).
Meanwhile dice the onion fairly finely and fry in a little oil or butter until gently brown, around 5-10min, stirring now and then.
While the potatoes and onions are cooking, butter all round a 6in wide souffle dish then swirl around approx 1 ½tbsp finely grated parmesan so that it sticks to the base and sides.
Once the onions have lightly browned, remove from heat and add the milk to the pan to allow it to take on the flavors as the onion cools.
After you have left it for a minute or two, peel the potato and coarsely grate them into the onion/milk mixture along with the grated cheddar and finely diced speck.
Lightly beat the egg yolks and add to the potato mixture, stirring to combine.
Whisk egg whites in a clean bowl until stiff.
Fold a spoonful or two of the whites into the potato mixture to loosen it then fold in the rest carefully (you can either do this in a separate bowl or carefully put the potato mixture into the side of the whites bowl before then folding in), folding the mixture just enough so combined and no big clumps of white. Carefully tip the mixture in to the prepared souffle dish and top with a little more finely grated parmesan.
Bake in oven around 25-30min until golden and it has risen well. Serve immediately.