Mix oil, soy sauce, lemon juice, Worcestershire sauce, mustard, garlic, salt & pepper.
Pour the mixture into a zip-top bag and add beef cubes. Toss to coat.
Seal bag & refrigerate 10 min. When ready to cook, preheat grill.
In a large bowl, pour boiling water over the mushrooms & let stand a few minutes.
Drain the mushrooms & thread onto skewers, alternating between marinated meat & mushrooms.
Grill over hot coals for 5-7 minutes with lid down.
Turn skewers and grill for 5-7 more minutes with lid up.