The best potatoes to use are ones with a fluffy texture – usually the type that roast well – sometimes referred to as floury potatoes(or mid to late season potatoes). Although you can make a potato ‘crush’ with new potatoes (also known as waxy or salad potatoes), you need floury (starchy) potatoes for a good purée.
Start by peeling the potatoes and cutting them into smallish, even-sized cubes; this way they will cook quickly and evenly and not become water-logged.
Put the potatoes in a large saucepan and cover them generously with cold water. Add some salt, then boil gently until tender. Drain well and return to the wiped-out saucepan. Put the saucepan back over heat for 30 seconds (without the lid); this drives off any clinging moisture, making the potatoes drier and improving the flavour(be careful not to scorch them though, or the flavour will be ruined). Alternatively, steam the potatoes until tender(my preference), remove steamer from the bottom pan of water and drape with paper towels and leave for a few minutes before mashing (the paper towels absorb excess water).
Mash with a hand-masher or pass through a mouli-légumes(my preference); a food processor makes them heavy and sticky. Heat a small amount of milk until just under boiling point (use as much as necessary to make a soft purée; I generally heat 1 cup/250ml of milk to begin for about 1 kg/2 lb potatoes). Add milk to the purée a little at a time, along with a generous knob of butter, beating well with a wooden spoon; hot milk makes the purée fluffy, cold milk added to a starchy vegetable makes it tacky and gluey. Hot milk also stops the purée from turning greyish, and of course, it keeps it hot. Beating it by hand introduces air and keeps the purée light.You can keep the potato purée quite stiff, for instance, if it is to be used on top of a cottage or shepherd’s pie, or make it softer and fluffier if it is to accompany a main course with sauce or jus or to be a fluffy pillow for saucy ingredients to rest on, or make it barely spoonable if you want it to provide a very soft almost sauce-like component to a dish. It’s all in how much butter and hot milk you add. To make it richer, add heated cream. In all cases, don’t boil the milk or cream, just get it piping hot, so that you keep its good flavour and don’t get froth or foam or a skin forming.
Check the seasoning, add salt, beat it in, then taste again and continue adding salt and tasting until the purée has a pronounced potato flavour and tastes simply delicious. This is the key to a tasty potato purée, and the place where so many go wrong. Salt makes it taste good. Simple as that.
The purée is best served immediately, though it can be kept for a short time. Don’t keep it over heat as that will do it no favours; it is likely to collapse, turn greyish or watery. Keep it off the heat, pour a little hot milk over the surface to prevent it drying out, cover with a lid and leave it to cool. When ready to reheat, lift off the lid, quickly inverting it so that any water on the inside of the lid doesn’t go into the purée, and set the pan over medium heat. Reheat, stirring with a wooden spoon, adding more hot milk if necessary.