The potato should be as starchy as possible and in the
I steam the potatoes as that yields tender and dry potatoes which extremely important. If too much of the starchy boiling water comes along with the potatoes the mash becomes too liquid and the glue like quality that mash sometimes has can also be caused for by the water. If you boil the potatoes in salty water start with hot water and after the potatoes are done but them back into the pot over low heat to dry off. Whichever method you use the most important point is to cook the potatoes to a point where they are tender but not falling apart. Never undercook the potatoes as they will become gummy when you mash them.
Ideally you should mash the potatoes immediately after cooking them as they are more want to go lumpy if you allow them to go cold. I prefer my mash quite rustic and grainy rather than the very smooth mash one usually gets in restaurants. This does not imply my mash is not smooth as I often push it through a sieve to get a finer texture. I however, never use a mixer or a blender for mashing although that is very much an accepted professional method. I always use the kind of specialised whisk that kitchen stores sell and quite possibly call a potato masher. The important tip about using the masher is to only use an up and down motion. If you make a circular motion you will pull out/extend the potato and get the glue like quality I mentioned above.
The next step is to add the fat, i.e. oil, milk cream and/or butter. All typical classical recipes call for milk or cream and butter I however have always preferred olive oil to milk or cream. Using cream make the mash fluffy and mousseline, which works in conjunction with some food such as pork neck and lobster. Milk will make for a less creamy but still rich, if less so, mash that is good with most food (certainly poultry and fish) but in my humble opinion not tasty enough to cope with a steak or game. That being said even when I use mostly olive oil I do add a little bit of milk.
The quantity to use varies a bit according to what the ultimate use of the mash will be and also how starchy the potatoes are which can vary depending on the time of year. Storage also matters as if the storage temperature is too hot sugar will convert to starch. You consequently need to heat more milk or oil than you expect to need and add it as needed. I use a rule of thump that you need 25% of the volume of the potato as milk/oil and about 100 grams butter per kilo of potato.
Whatever the source of the fat it has to be hot before use so scald the milk, and heat the butter and oil until warm but not boiling. You can add the butter at anytime so I tend to heat it with the oil. Make sure the potatoes are smooth before adding any liquid as you will not be able to get the lumps out afterwards.
You can season the mash at any time although I tend not to leave it too late as once you have mashed the potatoes you do not want to be turning them over to get the seasoning in. Nutmeg is a classic addition to mash and supposedly it removes the taste of storage that potatoes can have at the end of winter. I’m not a fan. Salt and white pepper are a must – always white pepper as black looks odd and is too strong. To this you can add almost anything. Truffles, boiled vegetables, mustard etc it all depends on what you will be eating with the mash.
Keep in mind that if you leave mash to sit the starch will cause it to become blander. So if you are not using the mash immediately make it tastier than you want it to be when served.