They don't appear to be close relatives, but onions belong to the lily family. So do the garlic, leek, chive, scallion and shallot bulbs. They contain "odoriferous sulfur compounds", responsible for flavor and eye-irritating compounds.
There are a variety of medicinal effects that can be found with a Google search -- treating coughs, colds, bronchitis, perhaps asthma, preventing atheroscerosis, suppressing growth of potentially harmful bacteria in the colon. Both onions and garlic provide sulfides which may lower blood lipids and blood pressure. One site stated: The higher the intake of onion, the lower the level of glucose found during oral or intravenous glucose tolerance tests.
The very smell of cooking onions whets the appetite. Whether raw, sauted, fried, simmered, or roasted, they add flavor to what they accompany.
We enjoy them in so many ways.
My favorite is to caramelize them and serve as a side to a favorite meat (ribeye steaks sound just about right!) Start with several large onions -- I find the sweet ones to work quite well, Texas' 1015 or Vidalias. Remove the root and top ends, then peel. Some say to cut the onions in half, from root to stem, then slice lengthwise. I prefer to make onion rings. Use a large pan to saute. Heat olive oil, butter or a combination (about one teaspoon for each onion. Once the oil is hot, add the onion sices and stir well to coat them. Some recipes advocate adding sugar -- I don't. Cook for 30 minutes or more. Stiring every few minutes is an absolute necessity -- they must brown, but not burn. So it's cook, turn, cook, turn (scraping up the pieces that begin to stick to the pan.) Everyone has a favorite way to deglaze the pan -- find your own: a small bit of water, balsamic vinegar, cooking wine, stock. Each has their own advocates. Use right away or they'll keep in the refrigerator for several days.
Another easy one, to eat immediately after cooking, is a baked onion. Cleanse the onion well, leaving the roots intact but spotless. Peel the onion and cutting just to (but NOT through) the root. Place onion on a lightly greased square of aluminum foil. Press butter into the cuts, then sprinke with your favorite seasonings: salt, pepper, fajita / cajun spices, cheeses, include a slice or two of bell pepper. Wrap in the foil, place in a baking pan, then bake at 400 degrees for about an hour. Excellent!