Especially if you're going to put your onions directly on your grill, you need it to be clean. Even if you're not, a clean grill cooks better than a dirty one.  To clean your grill, put some vegetable oil on a paper towel, and wipe each rod down, one by one. You can also put the oil directly onto the grill and scrape off the grime with a grill spatula or other tool if necessary. Some grills have a "cleaning" setting. If the above isn't enough, you may wish to use this option if you have it. Oil not only cleans your grills, but prevents food from sticking to it. Be sure to this before you turn the heat on, otherwise it will cook away. Any kind of oil is fine. This is not the oil that will be going on your onions, so a cheap vegetable oil will be perfect for the job. Barring circular slices of onion, it's easiest to grill onions with some sort of device to make sure all sides get cooked and that they don't fall through the grates. A tin foil tent, basket, tray, or skewer inserted through the onion will all work well.  You could also just use a piece of tin foil over the grate, with holes poked through it to let out heat. If you do use skewers, make sure they're soaked in water first so they don't catch on fire. If you have none of the above items on hand, you'll need to stick to large slices or whole onions. Large, circular slices are going to be easiest as it's difficult to grill a whole onion evenly – inevitably one side gets more cooked than the others. Olive oil, salt and pepper, and your favorite seasoning blend are staples of grilled onions. Some people also like to use butter and beef bouillon granules, too. Here are some other ideas you could use, regardless of the shape of your onion:  Honey mustard Balsamic vinegar BBQ sauce Worcestershire sauce Steak seasoning

TL;DR in Chinese:
清洗烤架。 清洁完毕后在烤架上喷油防止食物烤焦。 选择锡箔，烤篮，托盘或烤肉扦来烧烤洋葱。 如果手头没有上述器具，请将洋葱切片或整只烧烤。 选择烤洋葱的涂料。