Place beef in a 4 to 6-gallon heavy-duty plastic bag set in a rimmed baking pan (at least 10 by 15 inches).
In a bowl, stir together ale, oil, onion, parsley, garlic, bay leaves, and pepper.
Pour over roast in bag; seal bag securely.
Let stand at room temperature for at least 3 hours or refrigerate until next day, turning bag over several times.
Lift meat from marinade and drain briefly (reserve marinade).
Fold thin end of fillet under; tie securely with string to make fillet as evenly thick as possible.
Place meat on a lightly greased grill 4 to 6 inches above a solid bed of medium coals.
Cook, basting frequently with marinade and turning every 5 minutes for even browning, until a meat thermometer inserted in thickest part registers 135° to mot for rare (about 50 minutes).