In a bowl, combine all the ingredients. Add the beef tenderloin to a gallon freezer bag; pour the prepared marinade over the top into the bag; seal tightly.
2/3 cup good olive oil, 2 cloves garlic, crushed, 1 tablespoon dark brown sugar, 1/4 cup Worcestershire sauce, 1/4 cup soy sauce, 1/4 cup balsamic vinegar, 2 tablespoons spicy brown mustard, 2 tablespoons fresh rosemary chopped, 1 teaspoon black pepper
Place the bag of beef tenderloin with marinade into a large mixing bowl and into the fridge. (The bowl is good to have in case the bag happens to leak.) Let the beef tenderloin marinate overnight or 10-12 hours.
When you are ready to prepare the beef tenderloin, remove it from the marinade and pat the exterior of the meat dry with paper towels.
Cook the tenderloin as desired, either on the grill or roasted in the oven.
Notes
Marinade must be discarded unless you prefer to simmer it in a saucepan until reduced down into a sauce.