This backyard grilling staple is nothing short of delicious. But the pre-made kebabs from the grocery store leave us underwhelmed--with tough pieces of meat and chunks of vegetables that always seem to end up either burnt or undercooked. For our beef, we chose tenderloin, which chars beautifully and stays tender during cooking. The vegetables are cut small enough to cook through without burning or falling apart and the tangy marinade brings everything together.
Soak the skewers in water for at least 15 minutes and up to overnight. Pat them dry and spray lightly with nonstick cooking spray.
Whisk the vinegar, scallions, Dijon, garlic, sugar, Italian seasoning, red pepper flakes, 1/2 teaspoon salt and a few grinds of black pepper in a medium bowl or large liquid measuring cup. Whisking constantly, slowly drizzle in the olive oil until emulsified.
Put the beef pieces, bell peppers, onions and zucchini in a large resealable plastic bag. Reserve 1/4 cup marinade and pour the rest into the bag and seal. Toss the bag so the meat and vegetables are well coated. Marinate in the refrigerator for 2 hours and up to overnight.
Prepare a grill for medium-high heat.
Thread the meat and vegetables onto a skewer in the following order: beef, zucchini, red pepper, yellow pepper and onion. Repeat the threading three more times, ending with a piece of beef (there should be 5 pieces of beef and 4 pieces of each vegetable on the skewer). Repeat with the remaining skewers and ingredients.
Sprinkle the kebabs with 1/2 teaspoon salt and a few grinds of pepper. Grill at an angle, uncovered, until deep char marks form and the vegetables start to soften, 5 to 6 minutes. Flip the skewers and continue cooking until the beef has reached an internal temperature of 125 degrees F when measured with an instant-read thermometer, about 5 minutes.
Remove from the grill and brush the kebabs with the reserved 1/4 cup marinade. Let rest for 5 minutes before serving.