To prepare the fennel bulbs, first cut off the leafy fronds and reserve them for a garnish. Now trim off the green shoot by cutting diagonally to make a V-shape. Then slice off the root part at the other end, keeping the bulb intact, and remove any tough or brown outer layers, then slice across each bulb to cut it in half.
Then place the fennel in a fan steamer set in the saucepan with 2.5cm/1in of boiling water under it. Cover and steam for 10 minutes, then remove them from the steamer, throw out the water, wipe the inside of the pan with kitchen paper and return it to the heat.
Next melt the butter and sugar in the saucepan and when it starts to foam, stir it around the pan until the sugar dissolves, then add the fennel, cut side down. Keeping the heat fairly high, brown it for five minutes, then turn the pieces over and brown them on the other side for another three minutes.
Now combine the cider, cider vinegar and a little salt, and pour this into the pan; then, keeping the cut-side of the fennel facing upwards, cover with a lid and simmer gently for 20 minutes. After that turn the fennel over again. Then continue to cook for a further 20-25 minutes (this time uncovered). Watch carefully during the last 10 minutes and test to see if it is cooked by inserting a skewer.
When the fennel is tender enough, raise the heat so that the remaining juices reduce to a glaze. Shake the pan carefully to give an even covering of the caramel glaze. Now transfer the whole lot to a warm serving dish with the cut surfaces upwards and scatter with the chopped fennel fronds as a garnish.