Melt half the lard in a sauté pan placed over a medium heat. Add the lardons and cook until crisp. Add the apple the juniper berries and cook for a minute or two, until beginning to soften. Add the sauerkraut and pour in the beer. Bring to the boil and bubble until all the liquid has reduced. Set aside.
Lightly flour the mullet fillets. Heat a tablespoon of olive oil in a frying pan and fry the fillets, skin-side down for 2 minutes. Turn the fish over, turn the heat off and allow to the fillets to cook in the residual heat. Finish off with the remaining lard.
Pour the port into a small saucepan, bring to the boil and simmer until the volume of the liquid has reduced two thirds and is sticky. Stir in the remaining olive oil.
Stir the herbs into the sauerkraut and divide between two plates. Lay the mullet fillets on top and drizzle with the port reduction.