Cajun Butter Steak

In a large bowl, whisk together soy sauce, olive oil, brown sugar, bourbon, mustard, Cajun seasoning, garlic, thyme, and red pepper flakes. Pour out about ⅓ of mixture to use for glaze later. Toss steak in mixture and let marinate for 20 minutes, or up to 4 hours. (If marinating for more than 20 minutes, cover bowl with plastic wrap and refrigerate.) Heat grill or grill pan to high. Remove steak from marinade and season both sides with salt and pepper. Grill until charred on bottom, about 10 minutes. Flip steak, then lower grill to medium-high and cook for another 8 to 10 minutes. (A meat thermometer inserted into the thickest part of the meat should register around 130°.) Remove from grill and let rest on cutting board before slicing against the grain.  Brush glaze over steak and garnish with parsley, if using.