Tip: You can marinate the steaks in the refrigerator for up to 2 hours before grilling. Just bring the steaks back to room temperature before grilling.Prepare a medium-hot grill fire. Meanwhile, combine the soy sauce, bourbon, sugar, mustard, and hot sauce in a large zip-top bag. Seal and shake to combine the ingredients and
dissolve the sugar. Add the steak to the bag, seal, massage to cover the steaks with marinade, and set aside for 15 to 20 min. at room temperature.
When the grill is ready, remove the steak from the marinade and shake off any excess, but don’t pat it dry. Reserve the marinade. Grill the steak until good sear marks appear, 3 to 4 min. With tongs, rotate the steak 90 degrees (to get a crosshatch of grill marks) and continue grilling until grill marks form and the edges are a little crisp, another 3 to 4 min. Flip the steak and grill the other side in the same way until the exterior is nicely seared and the steak is cooked to your liking, 10 to 12 min. total cooking time for medium rare. Let the steaks rest for about 5 min.
Meanwhile, pour the marinade into a small saucepan and boil over medium-high heat until syrupy, about 3 min. (watch carefully to prevent burning). For flank steak, slice the meat thinly across the grain. For strip steaks, slice thickly or serve in chunks. Serve with a drizzle of the sauce.