
Use a mandoline or a very sharp knife to slice all 4 onions into even slices to about ¼- to ½-inch thick.
Heat a large, cast-iron skillet over low heat and add the olive oil. When the olive oil is fragrant, add the onions.
Toss the onions with olive oil and cook on low for 30 minutes, stirring periodically.
After 30 minutes, season the onions with salt. Stir and let the onions cook for another 20-30 minutes over low heat, stirring periodically.
When the onions turn a golden brown color, remove them from the pan.