For herbed kipflers, cover kipflers in a saucepan with salted water and boil over medium-high heat until kipflers are just cooked (10-12 minutes). Drain and set aside to cool (1 hour). Halve potatoes lengthways and place in a large frying pan over medium-high heat with oil and garlic and fry until golden and crisp (5-6 minutes). Add herbs and butter and toss potatoes to coat, then remove from pan with a slotted spoon and keep warm.
For béarnaise sauce, place vinegar, shallot, tarragon sprigs, bay leaf and peppercorns in a saucepan, bring to the boil, then reduce heat to medium and simmer until reduced by two-thirds (2-3 minutes). Strain through a fine sieve and set aside to cool (40 minutes; discard solids). Combine egg yolks and 25ml reduction in a heatproof bowl and place over a saucepan of simmering water (don’t let the bowl touch the water). Whisk mixture, rotating bowl occasionally, until pale and thick (2 minutes), then remove from heat and place bowl on a wet tea towel to hold it steady. Add clarified butter in a thin steady stream while whisking continuously until butter is incorporated and mixture thickens slightly. Season to taste, then fold in herbs and keep in a warm place.
Heat olive oil in a large frying pan over medium-high heat, season steaks and fry until cooked to your liking (4 minutes on one side, then turn and cook for another 2 minutes for medium-medium rare). Add butter to pan and cook, basting meat with butter as it melts, to your liking (2 minutes to cook steaks medium). Rest steaks on a rack over a baking tray in a warm place for 4 minutes before serving.
To serve, place steak and kipflers on plates, garnish with watercress and top with béarnaise.