Step 1 Put the butter in a small heat-proof casserole or Dutch oven -- preferably made of enameled cast iron -- and set it over gentle heat. Add the sage leaves and, when they soften slightly, after a minute or so, add the garlic and let it soften for 15 to 20 minutes without coloring. During this time, the butter should murmur very gently without burning.
Step 2 Meanwhile, cook the potatoes in their skins in a separate large saucepan in gently simmering salted water. Test after about 10 minutes. The potatoes should be easily pierced by a small sharp knife. Drain.
Step 3Spoon the potatoes gently into the flavored butter in the casserole and leave them to color gently and continue cooking for several minutes.
Step 4When ready to serve, season with salt and serve garnished with the sage leaves either as a first course or as an accompaniment.