Turkish Chicken Kebabs

Whisk soy sauce, vegetable oil, lemon juice, garlic, mustard, Worcestershire sauce, salt, and black pepper together in a bowl; pour into a resealable plastic bag. Add beef cubes and squeeze out excess air; seal the bag. Marinate in the refrigerator 8 hours or overnight.

Add mushrooms to the bag; squeeze out excess air and reseal the bag. Marinate in the refrigerator another 8 hours.

Preheat an outdoor grill for high heat and lightly oil the grate.

Remove beef and mushrooms from the marinade, shaking off any excess liquid. Pour the marinade into a small saucepan and bring to a boil over high heat. Reduce heat to medium-low and simmer for 10 minutes; set aside for basting.

Thread pieces of green bell pepper, beef, red bell pepper, mushroom, and onion onto metal skewers, repeating until all ingredients are skewered.

Cook the skewers on the preheated grill, turning frequently and brushing generously with the reserved marinade until nicely browned on all sides and beef is no longer pink in the center, about 15 minutes.