Slice the onions into half moons. Cut off and discard the stem end of the onion, then lay the flat end on the cutting board. Slice in half from root to tip. Peel away the outer layers of skin, place each half flat-side down on the cutting board, and slice into thin half moon strips, discarding the root ends. Repeat with the remaining onions. (Note: see my post for a video demonstration of this process).
Place a large stainless steel, aluminum or cast iron skillet (do not use nonstick) over low or medium-low heat. Once the pan is warm, add just need enough oil to create a thin coating on the bottom of the pan. Have a small liquid measuring cup with water nearby.
Add the onions along with a pinch of salt, and cook, stirring occasionally, until some brown bits begin forming on the bottom of the pan, 4-5 minutes. The less you stir, the faster the brown bits will start to form. However, you do want to stir occasionally for even cooking and to prevent burning.
Once the brown bits (aka fond or glaze) have accumulated and darkened but are not yet starting to burn, add a splash of water to deglaze the pan (1-3 tablespoons; just enough to loosen up the fond). Use a firm spatula or wood spoon to scrape up the brown bits and stir them back into the onions.
Repeat this process of glazing and deglazing for around 45 minutes. As the caramelization process moves further along, the process will speed up; you'll need to stir and deglaze more often.
Continue cooking until the onions and a rich brown color and they taste like a good balance of sweet and bitter.
Store in an airtight container in the refrigerator for 3-4 days, or in the freezer for up to 3 months.
You can use any type of onion (including leeks and shallots) with this method. I recommend using yellow onions for a neutral, balanced flavor.Use a firm spatula or wooden spoon to scrape up the brown bits (as opposed to something like a rubber or silicone spatula). This one is my personal favorite.