Heat oven to 325 degrees F. Set an oven-proof (cast iron) grill pan on a burner over high heat. Lightly brush steaks with olive oil on both sides and season with salt and pepper. When the grill pan is very hot, add the steaks. Sear the steaks over high heat for 3 to 4 minutes on each side. Remove the pan from the heat and set it in the oven to finish cooking. Keep the steak in the oven for 6 to 8 minutes for rare to medium-rare (or until an instant-read thermometer inserted in the middle of the steaks registers 165 degrees F). When steaks reach desired doneness, remove them from the oven and let them "rest" for another 4 to 6 minutes before slicing.

While the steaks are cooking, prepare the chimichurri sauce. Combine all of the ingredients in a food processor or blender and pulse until all ingredients are chopped fine and well combined, stopping to scrape down the sides, if necessary. The mixture should have the consistency of a coarse pesto. Set aside until ready to serve.

To serve, slice the steaks thinly against the grain and arrange on a platter. Drizzle some of the chimichurri sauce over the steak and sprinkle with toasted sesame seeds. Garnish the platter with pineapple rings, if desired. Transfer the remaining sauce to a bowl and pass with the steak.

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