Make a mixture of olive oil, vinegar, finely diced shallots and garlic to brush onto the kebabs. When cutting food for the kebabs, try to keep everything about one inch square. This is a good bite-sized portion. Do not worry about being exact, though. Vegetables are not perfectly square.
Red onions are the best to use because they have great flavor and less sodium.
However, keep the onion pieces on the small side. Separate the layers. A large hunk of onion on a skewer will not cook through, leaving the inside raw. Button mushrooms and cherry tomatoes are great kebab items that can be added whole. Lightly cook the outside of mushrooms before placing them on the skewer to prevent cracking.