The beauty of this savory marinade is that you can make it ahead of time, add the meat and onion, and then freeze it all together until you’re ready to serve.
In a medium-size bowl, whisk together bourbon, brown sugar, Worcestershire, and soy sauce.
Pour into a large zip-top bag; add onion and steak.
Seal, squeezing to expel any air, and let meat marinate at least 2 hours, up to overnight (or freeze up to 3 months).
When ready to cook, defrost steak and onions if you’ve previously frozen them; then drain and season meat with salt and pepper.
Set up your grill for direct high heat. To grill meat, sear on one side until nicely browned, 4–6 minutes. Turn meat and grill until it reaches desired doneness (4–6 minutes more for medium rare).
Cook onions in a grill pan, stirring occasionally, until golden, about 10 minutes. Or cook onions directly on grill grates away from direct heat, carefully turning once, for 8–10 minutes total cooking time.