Trim the beef fillet of sinew and fat.
Cut the beef lengthways into two flat pieces.
Season the beef with olive oil, salt, and peppercorns.
Heat a cast-iron pan over a high heat until very hot.
Sear the beef for 2 to 5 minutes on each side depending on desired doneness.
Transfer the beef to a plate and allow it to cool.
Combine celery, oil, salt, pepper, and 150ml of water in a small saucepan.
Cover tightly and simmer for 4 minutes.
Remove from heat and let the celery steep in the liquid.
Dissolve salt, pepper, and mustard in the vinegar in a bowl.
Slowly whisk in the oil to emulsify the vinaigrette.
Whisk 150ml of the vinaigrette with the crumbled Cashel Blue cheese.
Slice the beef thinly and arrange on plates with the celery.
Top with salad leaves and drizzle with the blue cheese dressing.
Garnish with fresh chives and chervil leaves.