Heat a cast iron or other heavy-bottomed pan until very hot. Spread the onions thinly across the bottom of the pan. The onions should sizzle as soon as they hit the pan. Sprinkle with salt and pepper. When the onions begin to blacken & caramelize (this will take 2-3 minutes depending on your pan), use a wooden spoon to stir them. Cook for 1-2 minutes more and then add jelly to the hot pan. Stir the onions until they become soft a deep golden brown.