Whisk pineapple juice, soy sauce, sugar, vinegar, and oil in a small bowl until sugar is dissolved. Add onion and ginger. Set aside 2 tablespoons marinade for butter. Transfer remaining marinade to a large resealable plastic bag. Add steaks and seal bag, pressing out excess air. Chill at least 1 day.
Remove steaks from marinade, pat dry, and let sit until room temperature, about 1 hour.
Butter and Assembly.
Whisk butter and oil in a medium bowl to combine. Vigorously whisk in reserved marinade; season with salt. Transfer to the center of a sheet of parchment paper and roll up into a log. Chill until firm, at least 1 hour.
Build a two-zone fire in a charcoal grill for direct (medium-high coals) and indirect (medium-low coals) heat. Alternatively, heat a gas grill to high just before cooking, leaving one burner on low. Grill over high heat, turning, until steaks are well browned and beginning to char around edges, about 4 minutes. Transfer to cooler part of grill and continue to cook until an instant-read thermometer inserted into thickest part of steaks registers 120°F for medium-rare. (Alternatively, cook in a grill pan over medium-high heat, turning steaks every couple of minutes.).
Transfer steaks to a wire rack. Slice butter into 1/4"-thick rounds and divide between steaks. Let rest at least 10 minutes before slicing.
Steaks can be marinated 3 days ahead. Keep chilled.