Preheat a griddle pan over high heat until hot.
Coat the steaks in olive oil, season, and griddle for 2 minutes per side.
Transfer the steaks to a plate and leave them to rest.
In a large bowl, combine the peas, broad beans, radishes, lettuce, and potatoes.
Mix the capers, gherkins, shallot, lemon, garlic, herbs, and rapeseed oil to create the tartare dressing.
Blend the oysters and their juice with mustard in a food processor until smooth.
Slowly drizzle in the vegetable oil while blending until the mixture thickens into mayonnaise.
Drizzle some tartare dressing on the steak and toss the remainder with the salad.
Serve the steak and salad with a side of the chilled oyster mayonnaise.