To Make the Steaks:
Heat the oven: Position a rack near the center of the oven and heat to 375 degrees (350 degrees convection). Let the steaks sit at room temperature while the oven heats.
Heat the skillet: Place a large cast-iron or black metal skillet (a 12-inch skillet will hold two steaks nicely) over medium heat and heat the skillet while you season the steaks.
Season the steaks: Sprinkle each steak aggressively all over with salt and pepper, turning the steak and pressing all sides down onto the seasonings that fall onto the work surface. You want the entire surface to be seasoned. If you prefer to measure, use ½ to ¾ teaspoon salt and ¾ to 1 teaspoon pepper per steak.
Sear the steaks: Once the pan is hot, increase the heat to high and add the oil to the pan, tilting to coat. When the oil begins to shimmer, after about 30 seconds, place the steaks side by side in the skillet. Let them sear without disturbing; nudging the steaks will interfere with the browning. After 2 minutes, lift the edge of one of the steaks to check whether its well seared. If so, immediately flip both steaks and smear the tops with butter, dividing it equally. (If the steak isnt browned yet, continue to sear for another 45 seconds and check again.)
Roast: Immediately transfer the skillet to the oven. After 6 minutes, start checking for doneness either by touching the meat (the steak firms up as it cooks) or by taking the internal temperature with an instant-read thermometer. Baste the steaks with pan juices each time you open the oven, and check again every 2 minutes until the steaks are done to your liking. Depending on what degree of doneness youre after and how often you open the oven to check on them, expect them to roast for 6 to 10 minutes, or until they reach 115 to 120 degrees internal temperature for rare, 120 to 125 degrees for medium-rare, and 125 to 130 degrees for medium.
Rest and Serve: Immediately transfer the steaks to a cutting board-preferably one with a trough-to rest for 5 to 10 minutes. Serve on individual plates if serving 1 per person, or cut in half to share. Pour the pan drippings and any juices from the cutting board over the top of the steaks and serve.
To Make the Steak Sauce:
Soak the citrus zest and raisins: Tear the orange and grapefruit zests into a few pieces each. Combine the zest, raisins, gin or water, and red pepper flakes in a small bowl. Cover tightly and let sit for 4 to 6 hours.
Soak the Tamarind Pulp: If using, put the tamarind ball in a small bowl, add ¼ cup warm water, and use your fingers to help dissolve some of it. Set aside to soften for 5 minutes. Work the tamarind with your fingers to dissolve more of the pulp. Let sit for another 5 minutes, then work again with your fingers to dissolve more pulp. Strain through a fine sieve into a small bowl, pressing on the solids. Set the tamarind juice aside and discard the solids.
Saut the Aromatics: Heat the oil in a small saucepan over medium heat. Add the onions, season with a pinch of salt, and saut until translucent, about 5 minutes. Increase the heat to medium-high, add the garlic, cloves, allspice, and cinnamon, and saut until fragrant, another minute or so. Stir in the tomato paste and sugar and cook, stirring frequently, until the bottom of the pan begins to darken, about 1 minute. Add the soaked raisins and zest, stir, and simmer briefly until almost evaporated, about 15 seconds.
Add the Liquid and Simmer: Add the grapefruit juice, orange juice, vinegar, and reserved tamarind juice (or tamarind or tart cherry juice concentrate, if using). Stir to combine, and adjust the heat so the sauce simmers steadily but doesnt boil violently. Continue simmering until the sauce is shiny and reduced by about one third, 15 to 20 minutes.
Strain and Cool: Pour the sauce through a fine-mesh strainer, pushing firmly on the solids to extract as much liquid as you can. Cool to room temperature and taste for salt. Transfer to a clean jar, cover, and refrigerate for up to 3 weeks.
