First, begin by caramelizing the onions. This is probably the most time-intensive part of the recipe, so I recommend that you make the onions beforehand — they’ll keep for a week or more in the fridge in a tupperware container. Start by heating 1 tablespoon extra-virgin olive oil in a 10- or 12-inch cast iron skillet over medium-high until the oil is very, very hot but not smoking. Add 2 large thinly sliced sweet onions and season to taste with coarse salt and pepper. The onions will be piled up high, probably to the rim, but let them cook undisturbed for 2 to 3 minutes.