How to Make It
Stir together butter and miso in a small bowl until well blended. Chill miso butter in an airtight container until ready to serve, up to 5 days.
Process pineapple juice, 1/2 cup water, kimchi, brown sugar, and ketchup in a blender until very smooth, about 25 seconds. Transfer to a large saucepan. Bring to a simmer over medium. Simmer, stirring occasionally, until thickened and reduced to about 1 cup, about 15 minutes (glaze will be grainy). Set kimchi glaze aside at room temperature.
Season steak all over with salt. Let stand at room temperature 30 minutes. Meanwhile, preheat grill to high (450°F to 500°F). Brush one side of steak with kimchi glaze; place steak, glazed side down, on oiled grates, and brush top with some of the kimchi glaze. Grill, uncovered, until glaze is caramelized in spots, about 12 minutes for medium-rare, turning and basting often with kimchi glaze. Transfer steak to a cutting board, and let rest 10 minutes. Cut steak against the grain into slices, and divide among 4 plates. Garnish steak with sesame seeds and scallions, and serve with your favorite grilled vegetables tossed in miso butter to taste.