Mix the balsamic vinegar, oil, honey, Worcestershire, Dijon mustard, garlic, salt, and pepper in a small bowl
Place the flank steak and marinade ingredients in the slow cooker and flip the steak once or twice to coat the steak.
Cook undisturbed for 6 hours on low or 3-4 hours on high until very tender.
Carefully transfer the steak to a cutting board, let it rest for 10 minutes, then slice it against the grain and serve.
Notes
Look for uniform thickness in the flank steak to ensure the most even cooking.
Cook on low for 6 hours to get the most tender steak.
Flip the steak in the marinade in the slow cooker once or twice.
Do not add water to the slow cooker.
Let the steak rest for a few minutes before slicing to help it retain all its juices.
Store leftover flank steak in an airtight container in the refrigerator for up to 3-4 days.
To reheat, warm the steak slowly in a pan on the stove with a splash of liquid. If microwaving, use 50% power and heat the steak in 30-second bursts until warmed through.
This nutrition information is based on the exact products I used in this recipe. Brands and sizes of products could alter exact nutrition and should always be calculated independently.