Preheat your grill to 350 degrees Fahrenheit. This is the perfect temperature for cooking the picanha roast to a medium-rare or medium doneness, which is ideal for this sandwich.

Slice the sweet onion thinly and set it aside. You can use a mandoline slicer to get even slices.

Score the fat of the picanha roast in a diamond pattern. This will help the fat render and create a nice crust when searing.

Season the picanha roast with salt, making sure to rub it evenly into the scored fat. This will add flavor and help the crust form when searing.

Sear the picanha roast over direct heat until it's nicely charred, then remove it from the grill. Searing the roast over direct heat will create a flavorful crust on the outside while keeping the inside tender and juicy.

Cut the picanha roast against the grain into 1/2 inch thick steaks. Cutting against the grain will help the meat stay tender and make it easier to eat.

Season the picanha steaks with salt, making sure to rub it evenly on both sides.

Sear the steaks again over direct heat for about 4 minutes per side, or until the internal temperature reaches 125 degrees Fahrenheit. This will give the steak a nice sear and cook it to a medium-rare or medium doneness.

Remove the steaks from the grill and let them rest for at least 10 minutes. Resting the steak will help the juices redistribute and make it more tender and flavorful.

While the steaks are resting, make the chimichurri sauce by mixing the chimmi seasoning, red wine vinegar, and olive oil in a bowl. Chimichurri sauce is a traditional Argentinean sauce made with herbs and spices, and it's the perfect complement to the rich, juicy steak.

Sauté the sliced onions until they are translucent. Sautéing the onions will bring out their sweetness and add a nice flavor to the sandwich.

In a large skillet, lay down your hoagie rolls and add a slice of Gruyère cheese to each one. Using a skillet to toast the hoagie rolls will give them a nice crust and melt the cheese.

Once the cheese is golden, remove the hoagie rolls from the skillet.

Slice the rested picanha steaks into thin slices, against the grain. This will make them easier to eat and give the sandwich a nice texture.

Add the sliced picanha steak to the hoagie rolls, top with arugula, sautéed onions, and a spoonful of chimichurri sauce. The arugula adds a peppery flavor and the chimichurri sauce adds a tangy, herbaceous note that complements the rich, juicy steak.

Serve immediately and enjoy your delicious grilled picanha steak sandwich! This sandwich is perfect for a summer BBQ or a quick and satisfying meal any time of the year.