Preheat oven (electric cooker: 200 °C/circulating air: 175 °C/gas: see manufacturer). Line a baking tray with baking paper. Dab pork tenderloin dry, season with salt and pepper. Heat oil in a pan.
Brown the meat for 2-3 minutes. Bake in a hot oven for 12-14 minutes.
In the meantime drain the tuna. Rinse anchovy, pat dry. Mix mustard, lemon juice, whole egg and olive oil to a mayonnaise. Add the tuna, anchovy and egg yolk and continue mixing. Stir in the stock until you have a smooth sauce.
Finely chop 1 tablespoon capers and stir in. Season with salt, pepper and 1 pinch of sugar. Wash the chives, shake dry and cut into rings.
Remove meat from the oven and let it rest for a short time. Then cut into thin slices, arrange on plates and drizzle with the sauce. Sprinkle with 1 tablespoon capers and chives.