Fill a large pot with cold water. Season with a generous amount of kosher salt. Add the potatoes to the pot and bring to a boil over high heat. 

Lower the heat and boil gently, with the lid slightly askew, for about 10 minutes.  

Add the cauliflower and cook for 10 to 15  minutes, or until a fork easily glides through the potatoes and cauliflower.

Drain the potatoes and cauliflower. Return the cooked vegetables to the empty pot set over low heat. Shake the pan to “dry” the potatoes. This step helps to absorb any excess water from the potatoes, allowing again for the fluffiest consistency.

Meanwhile, heat the cup of stock in the microwave for about 30 seconds or until warm. This is a great trick to know about (which also works for when you’re making bechamel). The general rule of thumb is that if you’re going to be adding more than a cup to mashed potatoes (or bechamel), heat the liquid so that it more easily and readily absorbs into an already hot mixture.

Using a hand held masher (not an immersion blender or a food processor, which will make your mashed potatoes “gluey”), start to mash the potatoes and cauliflower. Pour in half of the required liquid, mashing continuously. 

Add margarine, and mash in. 

Add the remaining liquid as needed, mashing until fluffy and smooth. Season to taste with salt and pepper.