In a small bowl or glass measuring cup, combine the brown sugar, bourbon, olive oil, soy sauce, thyme, garlic, pepper and red pepper flakes. Whisk to combine.
Place the steak into a rimmed pan and cover the steak with the bourbon steak marinade. Wrap loosely with plastic wrap and refrigerate, flipping occasionally to ensure uniform marinating. Conversely, you can use a resealable plastic zip top bag, place the steak and marinade in the bag and squeeze out as much air as possible before sealing it. Flip the steak around a few times to coat it with the marinade.
Place in the refrigerator for at least two hours or overnight, flipping and rotating the meat every few hours. (note: the longer the beef marinates, the more flavor will penetrate it.)
GRILLING EASY FLANK STEAK RECIPE:
Heat barbecue to a medium high heat, about 450°.
Remove steak from marinade and blot off the excess moisture with a paper towel.
Place steak on hot grill. Cook for 5-6 minutes on the first side for medium rare. Flip the steak and cook an additional 5-6 minutes.
SERVING GRILLED MARINATED FLANK STEAK:
Transfer steak to a cutting board and allow it to rest for 5 minutes so that the juices redistribute throughout the meat.
Slice the flank steak thinly, against the grain and serve.
Cooking times depend on the size of your steak, and your preference on doneness. We like ours medium rare.