Both Birds Eye and Green Giant have frozen cauliflower products that are good substitutes for potatoes and rice! Here's a bit of information about
them.
Birds Eye has plain riced cauliflower which is a good substitute for rice or quinoa. The really good news is that it's only riced cauliflower and it's 3 gr. carb per 3/4 cup serving. They also have riced cauliflower with Italian cheese, garlic or savory herb that have 4, 5 and 6 gr. carbs respectively. However these flavored products also have quite a bit of sodium whereas the original plain riced cauliflower does not.
Green Giant has several mashed cauliflower products: broccoli and cheese (8 gr. carb), cheddar and bacon (6 gr. carb), original mashed with olive oil and sea salt (7 gr. carb), and finally garlic and herb (8 gr carb). Their plain riced cauliflower has no additives at all and has 4 gr. carb per 1 cup.
There might be other brands jumping onto this wagon so keep your eyes on the frozen vegetable section for these products. Beware of the added 'flavors' though because of the sodium content.
You can rice a head of cauliflower yourself with a food processor. It takes a bit of time to do a whole head and of course there's the clean up.
And if you want mashed cauliflower (which I now make at Thanksgiving instead of potatoes), you can mash hot cooked cauliflower florets as you would potatoes and add what you want. Make sure you drain the hot, cooked cauliflower to get rid of most of the water. Add about
a quarter to a half a package of cream cheese, a little half and half and butter and use a mixer or rotary beater to whip it all up smooth.
The cream cheese blends well with the warm cauliflower mash and gives it a creamy texture.
Lately, I've cooked some riced cauliflower in a very small amount of water and then add some parsley and a pat of butter. This really does resemble rice or quinoa and is a good side. Lots of possibilities for creating your own combinations which would keep the sodium lower than the commercial products.
Enjoy.
(Edited to fix the mashed cauliflower recipe.)