place the steak in a resealable bag with half of the chimichurri, seal, then massage for 30 seconds. set aside in the fridge for 30 minutes and up to 4 hours.
preheat grill to 450F to 500F.
add the thinly sliced zucchini to the steak bag to toss in the chimichurri sauce. place both the zucchini and marinated skirt steak on the preheated grill to cook 3-4 minutes on each side, for medium rare, or 4-5 minutes on each side, for medium.
let the steak rest for 5-10 minutes before thinly slicing against the grain for tenderness.
slice the ciabatta in half, generously slather both sides with the herby butter, then toast under the broiler until browned, 1-2 minutes.
layer the bottom slice with teh grilled zucchini, thinly sliced steak, remaining chimichurri, and baby arugula. sandwich with the top slice to enjoy!