Preheat the oven to 220°C.
Blitz garlic in a blender, then pulse with rocket and mustard for 5 seconds.
Stir in horseradish, lemon juice, and olive oil, then season and chill.
Heat olive oil in a roasting tin in the oven and season the beef.
Sear the beef in the tin over a moderate heat until brown on all sides.
Sprinkle with thyme and roast in the oven for 5 minutes.
Add balsamic vinegar to the beef and coat in the caramelizing juices.
Melt butter in a pan and stir-fry mushrooms and garlic until tender.
Add cherry tomatoes and stir-fry for 2 minutes.
Add cognac, flambé the mixture, and remove from heat.
Lay salad leaves on plates and top with some mushroom mixture.
Thinly slice the beef, pile onto the salad, and drizzle with dressing.