Combine the ingredients for the marinade in a food processor and blitz until smooth. Pour over the steaks, toss to coat then cover with clingfilm and refrigerate for 3 to 4 hours or overnight if possible.
Preheat the oven to 160 degrees Celsius or 325 Fahrenheit. Drizzle the bulb of garlic with a little olive oil, wrap in foil and roast for 25 to 30 minutes until soft.
Squeeze the flesh from the garlic bulb and mix with the butter, anchovies, horseradish and tarragon. Season to taste then spoon onto a sheet of clingfilm. Roll into a sausage shape, wrap tightly and refrigerate until needed.
For the salsa, heat a little olive oil in a large frying pan or wok, add the spinach and cook for 20 to 30 seconds until wilted. Add the remaining ingredients, stir to combine then remove from the heat. Season to taste and set aside.
Bring a large pan of salted water to the boil. Peel the cassava, cut into chips then boil for 8 to 10 minutes until softened but still holding their shape. Drain thoroughly and leave to steam dry for a couple of minutes.
Heat a large, heavy based frying pan or griddle over a medium-high heat. Drizzle the steaks with a little oil and season with plenty of salt and pepper.
Cook for 2 to 3 minutes on each side (or until cooked to your liking) then transfer to a plate to rest for 5 minutes.
While the steaks are cooking heat the oil for deep frying to 180 degrees Celsius or 350 degrees Fahrenheit. Fry the chips in batches for 2 to 3 minutes until golden brown then drain on kitchen paper. Season with sea salt whilst still warm.
Serve the steaks with the chips, spinach salsa and slices of the garlic butter.