Here's the trick we use for caramelizing frozen onions... Use an oil with a fairly high smoke point... (safflower and sunflower oil have the highest... around 500 degrees F) and , using a heavy pot or skillet (cast iron) bring the oil to a hot temp... put the amount of frozen onions you need into a good size bowl and, using the bowl as a shield, dump the onions into the oil.
Key point; be sure to be ready with one of those perforated "splatter guard" lids and place it on the pot immediately. The splatter guard will let out the steam from the frozen onions and the hot oil will caramelize them fairly quickly... be sure to watch them to prevent scorching. After the initial release of steam and splattering water, stir the onions with a wooden spoon (just a foible of mine) and remove when they reach the color you want. Luck!