Make the Marinade: Cut the onion in half. Grate one half and place it in a medium bowl. Add olive oil, vinegar, lemon juice, garlic, mustard, thyme, oregano, salt, and pepper. Whisk until combined. Cut the other half of the onion into 1 ½-inch squares and set aside for skewering.

Marinate the Beef: Place beef cubes in a resealable bag or bowl. Pour in half of the marinade, seal, and refrigerate for at least 30 minutes or up to overnight. Reserve the remaining marinade for basting vegetables.

Assemble the Kabobs: If using wooden skewers, soak them in water for 30 minutes. Remove beef from marinade and discard used liquid. Thread beef onto skewers. On separate skewers, add vegetables—grouping them by type. Brush vegetables with reserved marinade.

Grill the Kabobs: Preheat grill to medium-high heat and oil the grates. Grill beef and vegetables for 8–10 minutes, turning halfway through. Remove each skewer when ingredients reach desired doneness.

Broil in the Oven: Preheat broiler on high and line a baking sheet with foil. Arrange kabobs on the sheet, broiling for 8–10 minutes and flipping halfway through.

Cook on the Stovetop: Heat a grill pan or skillet over medium-high heat. Add a little oil, then cook kabobs for 8–10 minutes, flipping halfway for even searing.

Serve and Enjoy: Arrange kabobs on a platter and serve hot, either on their skewers or slid off onto plates. Pair with sides like rice, couscous, or Easy Dutch Oven No-Knead Bread for a hearty meal.