Best thickener for pureed vegetables

The idea of using boiled potatoes sounds great! But, I would suggest chickpea flour. Just roast chickpea flour for few minutes. Don leave it unattended, easily you can burn it. We just want to remove the raw flavor of flour by roasting it. Add it to the spinach. I do this when I make Indian dish " Palak Paneer" - which cottage cheese in Spinach. Please try this with a little bit of stuff before taking it out for potluck. Hope it works for you. Chickpea flour is gluten-free.

I've experimented with this same issue at a restaurant I work at. The potato method doesn't quite give it the body it needs. I've had the best luck with a little heavy cream or egg yolk.

I would thicken a spinach dish with cheese--lots of swiss cheese or maybe a nutty tasting parmesan...assuming the dish isn't vegan.

This recipe might be a helpful point of reference (sub for the flour, or skip): http://www.food52.com/recipes/561_polenta_creamed_spinach_a_healthier_take_on_a_steakhouse_classic

Certainly you can use corn starch, or get wild and toss some egg yolk into the spinach and cook in a pot (stirring all the time) or bake in the oven until the egg sets.

I rarely cook or bake gluten free but have on occasion and in this case I think you could use a gluten free flour like rice or corn or potato If you want to stick to the same profile that you usually make. I am sure it would work just fine. If you don't have any gluten free flour I am wondering if boiling a potato and puree with the spinach might work. Thinking outside the box here.

To clarify , i make a pureed spinach dish in which i usually use a roux to keep the spinach from separating. I am taking this dish to a potluck supper and want it to be gluten-free and reheatable.