Step 1

For chimichurri, place all ingredients except flank steak into a bowl, stir to combine. For the steak, cover both sides of the steak with some of the chimichurri, put in ziplock bag and refrigerate for one or two hours. Cover and refrigerate the rest of the chimichurri.

Step 2

Remove steak from refrigerator approximately 45 minutes before you are going to cook it. Scrape off some of the chimichurri, salt the steak and let rest. While steak is resting prepare the other ingredients: slice tomato, slice bread in 1-inch thick slices, mix the mayonnaise and chimichurri.

Step 3

To sear the steak: First with a paper towel pat the steak to remove any excess moisture. In a very hot fry pan lay steak, let it sear without disturbing it. Cook approximately 3-4 minutes on each side for a medium rare steak. (You will have to judge exactly how long to sear based on the thickness of the steak.) Remove from pan, lay on cutting board, pepper the steak now, let rest 5-10 minutes.

Step 4

While steak is resting, heat a fry pan (if you have one with ridges it will make nice grill marks on the bread). Spread butter on each side of bread, place in hot pan and grill, about a minute per side.

Step 5

When steak has rested, slice thinly, cutting across the grain. To put your sandwich together, spread the chimichurri mayonnaise on each side of bread, lay meat on bread, then add the tomato, put the other slice of bread on top and enjoy.