Add the bourbon, brown sugar, balsamic vinegar, olive oil, mustard, garlic, rosemary, & crushed red pepper flakes, if using, to a bowl or jar. Season with 2 teaspoons kosher salt & ground black pepper, as desired. Whisk or shake to combine.
Pour the bourbon marinade over your meat or veggies of choice. Marinate at least 6 hours or up to overnight.