Start by whisking together the soy sauce, vegetable oil, lemon juice, minced garlic, prepared mustard, Worcestershire sauce, salt, and cracked black pepper in a mixing bowl. The combination of tangy, savory, and aromatic ingredients forms the base of the marinade.

Place the beef cubes into a resealable bag or container and pour in the marinade. Seal the bag, removing any excess air, and give it a good shake to ensure the meat is evenly coated. Refrigerate for at least 8 hours or overnight for the beef to absorb all the delicious flavors.

Add the mushroom caps to the bag once the beef has marinated for a few hours. This ensures the mushrooms soak up just the right amount of marinade without becoming overly soft or breaking apart.

Before grilling, remove the beef and mushrooms from the marinade, but don’t toss it out! Pour the leftover marinade into a saucepan and bring it to a full boil for about 10 minutes. This sterilizes it, turning it into a safe and flavorful basting sauce.

Thread the beef cubes, mushrooms, bell pepper chunks, and onion squares onto skewers. Alternate the ingredients to make colorful and visually appealing kabobs.

Preheat your grill or grill pan to high heat and lightly oil the surface to prevent sticking. Place the assembled kabobs onto the grill, turning them frequently while basting with the boiled marinade. Grill for about 15 minutes or until the beef is fully cooked with a slightly charred exterior.

Transfer your grilled kabobs to a platter and serve hot. Pair them with your favorite side dishes, and enjoy every savory, smoky bite.