Is it a myth or does it really work?

Onion (Allium cepa) possesses expectorant properties and has effective germicidal properties. On account of its content of a volatile oil, the onion is very useful in respiratory disorders. The onion dislodges mucus and prevents its fresh formation. It liquifies phlegm and prevents its further formation. It has been used as a food remedy for centuries in cold, cough, bronchitis and influenza. Onions relieve nasal and chest congestion. Onions also induce perspiration. Several anti-inflammatory agents in onion render them helpful in reducing the severity of symptoms of the respiratory congestion associated with the common cold. Onion contains compounds that inhibit lipoxygenase and cyclooxygenase (the enzymes that generate inflammatory prostaglandins and thromboxanes), thus markedly reducing inflammation. Onions’ anti-inflammatory effects are due not only to their vitamin C and quercitin, but to other active components called isothiocyanates. These compounds work synergistically to spell relief from inflammation.

Onion Tea

Onion tea has mostly been forgotten these days, although it is a very simple and effective remedy for viral respiratory illnesses over the winter.
There are several ways of preparing the Onion tea, you can invent your own method. My grandmother used to make it using 1/2 pint of water and the outer dry (red) layers of 2-3 bigger onions. She boiled it for a minute and then set it aside covered for 15 minutes. When it cools down a little, add 2 tablespoonfuls of honey (Linden or Thyme honey is preferred if available).

Onion Cough Syrup

Red onions are also effective in relieving the symptoms of cold when used fresh.
If you need an instant cough syrup, simply press the juice of a bigger fresh onion and mix it with a teaspoonful of honey and drink. It is recommended to take frequent small doses, because larger doses may upset your stomach.

To achieve similar results without pressing, slice up your red onion and put the slices in a jar. Cover with sugar or honey and let it sit overnight. It will extract most of the onion juice by the morning. Take it fresh by the teaspoonful 3-8 times a day. It will be most effective when used fresh.

Onion Poultice to break up respiratory congestion

This is a compound poultice made of partially fried onions, cornmeal, and vinegar, and it’s an excellent way to break up even a stubborn congestion relatively quickly.
Mince the onion finely and put in a pan adding a little oil to it. Stir and saute it a little bit, then add a cup of vinegar, mix, then add cornmeal until it forms a thick paste. Spread your hot paste onto a muslin cloth and fold over to hold poultice and heat in. Apply it as warm as possible on the chest/throat area. Cover with a blanket or towel to keep it warm. Leave it on for a while, at least 15 minutes.