Preheat oven to 220C. Brush steak with half the olive oil, scatter evenly with pepper, pressing to coat and season to taste with sea salt flakes.
Heat remaining oil in a large ovenproof frying pan over high heat. Add 40gm butter and when foaming add steak and fry until browned (about 2 minutes each side). Place pan in oven and roast steak until cooked to your liking (7-8 minutes for medium rare). Transfer steak to a tray, cover loosely with foil and rest for about 8 minutes. Meanwhile, place frying pan over medium heat and deglaze pan with brandy, scraping up pepper from the base of the pan. Add 100ml stock and reduce to a light sauce consistency (1-2 minutes). Add 40gm butter a piece at a time, whisking to combine between additions, season to taste and keep warm.
Heat remaining butter in a large frying pan and sauté garlic and shallot until translucent (2-3 minutes). Add peas and remaining stock, and simmer until peas are bright green (2-3 minutes). Add lettuce and tarragon, toss to combine, season to taste and serve with sliced steak and pepper sauce.