Combine the soy sauce, olive oil, mustard, brown sugar, bourbon, garlic, Cajun seasoning, thyme, and rosemary in a large bowl until the sugar is dissolved. Reserve ⅓ cup to make the glaze.
Place the steak into the bowl and pour the marinade over it. Cover with plastic wrap and refrigerate from 30 mins to overnight. You can also place it in a zipper bag, and take out as much air as possible so the marinade can better penetrate the meat.
Preheat the grill, griddle or grill pan over high heat.
Remove the steak from the marinade and grill for 10-12 minutes on each side or until the internal temperature of the meat is 130 F on the thickest part.
Make the glaze: place the reserved ⅓ cup of marinade in a small saucepan and bring to a boil, reduce the heat and simmer until slightly thick. Remove from heat and stir in the butter.
Let the meat rest for about 10 mins. Brush with the glaze before slicing and serve the remaining glaze on the side.