In a small dish, mix together the salt, pepper, oregano, and garlic powder. Set aside.
Cut the zucchini into 1-inch thick slices and cut in half, set aside. Cut the onion into 1 to 2-inch pieces.
Cut the chicken into 2-inch cubes.
Skewer the chicken, zucchini, onion, and tomatoes, alternating each. Place on a platter.
Using a pastry brush, brush the skewers with the olive oil and sprinkle the spice mix on top and rotate to season each side. Squeeze lemon juice over the tops.
Heat grill to high, about 350°F to 400°F. Place kabobs on the grill and cover. Cook for about 3 minutes, rotate, and continue to do that until all sides have been grilled. Heat until internal temperature is 165°F. (Alternatively if you’re not grilling, preheat the oven to 400°F. Heat a cast iron skillet to medium/high. Add kabobs and sear each side. Transfer to the oven and cook for 8 to 10 minutes, until internal temperature is 165°F.)
Serve with tzatziki sauce.