For the herbed butter: Add the butter, chives, oregano, parsley, lemon zest, garlic and some salt and pepper to a bowl and mix together well.
Put the butter in the center of a piece of plastic wrap and, using the plastic wrap, form it into a sausage shape. Seal it and pop it in the fridge for 1 hour.
For the steak: Allow the steak to rest at room temperature for about 15 minutes. Fill a saucepan with some water, add a generous pinch of salt and bring to a boil. This is for cooking the string beans so it's all ready at the same time.
Season both sides of the steak well with the chopped rosemary and some salt and pepper.
Heat a cast-iron skillet over high heat until smoking. Add the oil and allow it to heat up for 1 more minute. Add the steak to the hot skillet and cook about 4 minutes per side for medium-rare. Remove to a plate, cover with a piece of aluminum foil and allow it to rest for 5 minutes.
For the green beans: Meanwhile, add the green beans to the salted boiling water and cook until bright green and crisp-tender, 3 to 5 minutes. Drain well.
When ready to serve, toss the green beans with about 2 tablespoons of the herbed butter. Slice the steak and top with a piece of the herbed butter.