Heat an ovenproof griddle pan until very hot. Season the beef fillet. Add the olive oil to the pan and then add the beef and brown well for 10 minutes on all sides until you have achieved a nice, thick crust.
Remove from the heat and leave to stand in a warm place until the beef has relaxed. This will take at least 10 minutes, but you could leave the beef to stand for up to 2 hours, provided it is not in too hot a place.
To make the horseradish cream, place the mayonnaise in a small bowl with the creamed horseradish and lemon juice. Season to taste and mix until well combined. Cover with cling film and chill until needed.
To make the crispy onion rings, heat the oil in a deep-fat fryer to 180C/350F, until a small piece of white bread turns golden brown in about 30 seconds; or use a deep-sided pan and ensure that it is only half way full.
Place the flour on a plate and season generously, then use to lightly dust the onion slices, shaking off any excess. Deep-fry for 4-5 minutes or until golden brown. Spread out on to kitchen paper and allow to cool and crisp up by lifting them occasionally to separate.
To serve, when the beef has rested, transfer it to a carving board. Use a very sharp carving knife to cut it into slices. Place the mixed salad leaves with herbs in a bowl and add enough of the dressing to lightly coat the leaves.
Arrange the beef on serving plates with a dollop of the horseradish cream, the dressed salad, a small pile of the crispy onion rings and an oven-dried country tomato.