1 tablespoon kosher salt
2 teaspoons packed light brown sugar
1 teaspoon finely grated lemon zest (from about 1 medium lemon)
1/2 teaspoon ground cumin
1/2 teaspoon paprika
1/4 teaspoon freshly ground black pepper
1/4 teaspoon ground coriander
1/3 cup olive oil
2 tablespoons tomato paste
5 medium garlic cloves, finely chopped
1 (1-1/2- to 2-pound) sirloin steak, about 1 inch thick
8 ounces cremini mushrooms
1 pound summer squash (zucchini, yellow, or a mixture of both)
1 medium red onion
10 (12-inch) wooden or metal skewers (soak wooden skewers in water for 30 minutes to avoid burning on the grill)
1.Place the salt, sugar, zest, cumin, paprika, pepper, and coriander in a large bowl and whisk to combine. Add the olive oil, tomato paste, and garlic and whisk to combine.
Transfer 1/4 cup of the marinade to a large zip-top bag, and set the bowl with the remaining marinade aside.
Trim the beef of excess fat and sinew and cut it into 1-inch cubes. Place the cubes in the bag with the marinade, seal it, and, using your hands, massage the meat to coat it with the marinade. Set it aside at room temperature to marinate for at least 30 minutes, flipping the bag halfway through. Meanwhile, heat a gas or charcoal grill to medium high (about 375°F to 425°F) and prep the vegetables.
Clean the mushrooms, trim the stems, and add the mushrooms to the bowl with the remaining marinade. Cut the squash into 1-inch chunks and add them to the bowl. Peel and cut the onion into 1-inch chunks, separate the layers, and add them to the bowl. Stir to coat the vegetables with the marinade, then thread them onto 6 skewers, leaving about 1/8 inch of space between each piece; set aside.
When the beef is ready, remove it from the bag and thread it onto the remaining 4 skewers, leaving about 1/4 inch of space between each piece.
Place the skewers on the grill. Cover the grill and cook, turning the skewers every 2 to 3 minutes. Remove the beef skewers when all sides of the meat are lightly charred on the edges and an instant-read thermometer inserted into a piece registers 125°F to 130°F for medium rare, about 6 to 8 minutes total. Transfer the skewers to a serving platter and cover loosely with foil.
Continue to turn the vegetable skewers until all sides are lightly charred on the edges and the vegetables are crisp-tender, about 6 to 8 minutes more. Transfer the vegetable skewers to the serving platter and serve immediately.