The thick and watery variation in cervical mucus is extremely normal. If you want a comprehensive explanation, google "eggwhite cervical mucus" or EWCM, the term for it when it is thick. That happens once a month, at the middle of your menstrual month about two weeks before your next period, for a few days. It does that because that is when you ovulate, and the body is sending friendly, slippery, well aligned mucus to assist sperm that might need help coming up to see the egg. EWCM is clear enough to read a newspaper through, and as you said thick, and strong enough to hold together like raw eggwhite -- you could cut it with scissors. The usual vaginal secretions are more like watery milk, thinner, and dry pale yellow or pale greenish if dried on your underwear. They mesh together under a microscope, in order to bar entry to sperm (though, don't count on this for birth control) as opposed to EWCM that lines microscopically up to provide a highway for sperm. Some times of the month you don't have much vaginal secretion at all, like right before your period.
The onion odor might be from your upper inside of your legs, where they join the body, not from inside your vagina. There are some sweat glands in that spot that seem to transmit straight from the digestive tract, and it's especially noticeable if you've been eating onions or garlic. This has to do with our biological heritage, and how millenia ago before humans had the power of speech, their sweat was useful in signalling to each other about food sources and health. (If you want to read an interesting book about this, check out "Our Kind," by Marvin Harris.) Anyway, to be sure it's on the skin and not in the vagina, you might take a clean Q-tip and insert it after you've had a bath. My guess is that it will smell totally innocuous.
If you ever get a "dead meat" smell that you are sure is coming from in the vagina, look for a lost tampon from the previous period. But that does not sound like what you're describing at all. Use a good soap on the upper legs that is designed to cut skin oils, and the onion smell should wash up fine.