Whisk oil, garlic, thyme, lemon zest, salt, and pepper together in medium bowl. Combine beef and 2 tablespoons of marinade in 1- gallon zipper-lock bag and toss to coat; press out as much air as possible and seal bag. Toss zucchini and bell pepper with 2 tablespoons of marinade in second bowl and cover with plastic wrap. Refrigerate beef and vegetables for 1 hour, flipping bag after 30 minutes.

Stir parsley and lemon juice into remaining marinade and let sit at room temperature while beef and vegetables marinate.

Remove beef from bag, pat dry with paper towels, and thread tightly onto three 12-inch metal skewers. Thread vegetables onto three 12-inch metal skewers in alternating pattern of zucchini and bell pepper.

4A. For a charcoal grill Open bottom vent completely. Light large chimney starter mounded with charcoal briquettes (7 quarts). When top coals are partially covered with ash, pour evenly over center of grill, leaving 2-inch gap between grill wall and charcoal. Set cooking grate in place, cover, and open lid vent completely. Heat grill until hot, about 5 minutes. 4B. For a gas grill Turn all burners to high, cover, and heat grill until hot, about 15 minutes. Leave primary burner on high and turn other burner(s) to medium-low.

Clean and oil cooking grate. Place beef kebabs on grill, directly over coals if using charcoal or over hotter side if using gas. Place vegetable kebabs on cooler part(s) of grill (near edge of coals if using charcoal). Cook (covered if using gas), turning kebabs every 3 to 4 minutes, until beef is well browned and registers 120 to 125 degrees (for medium-rare), 12 to 16 minutes. Transfer beef kebabs to serving platter and tent with aluminum foil. Continue to cook vegetable kebabs until tender and lightly charred, about 5 minutes.

Remove beef and vegetables from skewers using fork. Serve, passing sauce separately.