Season the steak. With a sharp knife, trim any remaining sinew. Season the steak generously with salt and set aside while you prepare the other ingredients. The steak can be seasoned up to 1 day in advance (see tip).
Make the tonnato. In a blender or food processor, blend all of the ingredients except for the oils until smooth. Slowly stream in the oils. Taste for lemon, salt and pepper and adjust the seasoning.
Cook the steaks. Pat the steaks dry. Heat a cast iron skillet or stainless steel pan over medium-high heat for 2 minutes, until very hot. Pour a very thin layer of oil into the pan and wait until it shimmers. Sear the steaks for 3 minutes, or until a deep golden brown crust forms. Flip the steak and cook to medium rare (~125°F). Transfer the steaks to a plate and allow them to rest for at least 10 minutes before slicing.
Cook the broccoli. Wipe out the skillet and heat over medium heat. Toss the broccoli with a little neutral oil, salt, and pepper. Arrange on the skillet in an even layer. Cook for a few minutes on each side until the florets are a bit charred and the stalks are tender but still toothy.
Plate. Slice the steak against the grain. Arrange the broccoli and steak on the plate. Sprinkle the steak with flakey salt. Drizzle with tonnato sauce. Finely grate the horseradish over the plate (a microplane grater is perfect for this).