Heat a cast-iron skillet or pan over high heat. Melt the butter and add the sliced onions. Fry for 2–3 minutes on high heat, then reduce the heat to medium-low and continue cooking until the onions begin to caramelise, stirring occasionally. Stir in the two teaspoons of honey and sprinkle with salt, then continue cooking until the onions are golden and caramelised. The whole process will take around 20 minutes. Remove the caramelised onions from the pan and set aside.