When I cut out grain, I first found it hard to try and find alternatives.
 I have now replaced them with a number of products that work for me.

To replace potatoes in purees and mashes I use green plantains.  The starch
in them is apparently much less quickly assimilated and has a much better
nutritional profile than potatoes or similar products.  I also use a
combination of plantain flour, flaxseed meal, and almond flour as a base to
make a bread substitute,  I can alter the percentages and add coconut flour
to get a sweeter bread.

Turnips work well in stew as a starch component.