Yes this is a simple recipe, but sometimes the simplest things are the best. This is one of one all time favorite, my true comfort food. I always prefer the combination of sweet potatoes and regular potatoes. You can play with the ratios, but just sweet potatoes puree does not have the best smoothness, body and lightness. To make it fancier (for holidays and such) I added a bit more sweetness with caramelized onions and the Amaretti cookie crumbles. At the end of long day this will always bring comfort.
Peel the onion and slice in quarters. Slice each quarter to very thinly. Put the olive oil in a pan and warm it. Add the slices of onion and a bit of salt and satue on low heat for good 20-25 min or until it gets very sweet and has some brownish color.
In the meantime peel the sweet potatoes and the potatoes and cut them to rounds\pieces of about 2-3 inch thick. Put in a pan, add the cup of milk, water to cover and teaspoon of salt. Bring to a boil, lower the heat and cook for 20 minutes or until soft but not falling apart.
Drain the liquid and put the potatoes pieces back in the pan and on the heat just for a minute so they will get a bit drier. Take of the heat.
Add the butter to the pan and mash the pieces with potato masher or food meal if you have one, until smooth. Add 1/2 a cup of heavy cream, teaspoon of salt and 1/4 teaspoon of pepper. Mix well. If it looks dry add a bit more cream, but do not make it to watery.
To make it very smooth and pillowy push the mash potato through a fine sieve with a big spoon. It is a bit of worth the effort. You get much delicate and smooth puree (I like it better without the strings of the sweet potatoes). Taste for salt, due to sweetness it needs quite a lot.
If it gets a bit cold while working with the sieve, add a bit more cream and heat it a bit over a pan with boiling water or in the microwave.
Plate and decorate with the sweet caramelized onions. Crumble the Amaretti cookies and sprinkle. Drizzle a bit of olive oil if you want.