Make sure you use broth from bones, and you can always let it boil for a little while so it thickens on its own. That's what I usually do.

How about pureed potato or potato starch?

Non grain thickeners: Tapioca starch or arrowroot - both pure root vegetable starches.

Or potato starch (or potatoes or other root veggie like parsnip - cooked and mashed)

Easy to make a 'gravy' for your stew.

While cooking your stew, toss in a few onions (2-3) whole

then take out some of the liquid/broth when its about ready, 4 cup'ish (most of the liquid), remove the onions

liquify it and put back in the pot to cook it down. Usually with the good stuff from bones you can get the gravy to almost gel like a gravy. I do it with chicken all the time.

For a creamy gravy, just add some paleo sour cream (I make from my yogurt).

Tapioca starch works great. It takes a small amount--you'll have to google the substitution rate. Use it the same way as corn starch: mix with cold water until totally dissolved, then stir into the stew and bring to boil. It actually works better than corn starch, which breaks down after prolonged heating or refrigeration. Be careful not to use to much, because tapioca starch can get rubbery/stringy. Start with a little--you can always add more. Once it's in there, you're stuck.

Or you can borrow a technique from pureed soups, and remove a couple cups of the finished stew vegetables/liquid to a blender, puree, then add back to the pot. Any root vegetables like potatoes, carrots, etc.--even the aromatics like celery and onion--work really well this way.

I'm not a fan of gelatin as a thickener for hot things, because gelatin is really only thick as it cools. It's pretty watery when hot.

I'm also not a fan of using reduction on a soup or stew--I find it makes their flavor much too concentrated for my taste. Reduction works best for sauces, where you want powerful, well-developed flavor in a small amount of liquid.

Reduction http://en.wikipedia.org/wiki/Reduction_(cooking)

I puree mushrooms, onions and celery from the pot or use coconut flour (make sure you blend it in with a bit of the broth to make it smoother).Tomato paste also works.I have heard chia seeds too.

Some ideas to thicken up stews depending on the flavor you want:

1) gelatin

2) tomato paste

3) 100% canned pumpkin - even a starchier squash like acorn and butternut will do

4) sweet potato starch

5) potato starch

Stew extra carrots, onions, and celery. Once it's cooked, take some of it out and use an immersion blender to puree the veggies. Pour it back into the stew and simmer for a while.

I've had this question about tapioca starch - I used to use it when I was just gluten free (allergy) but since going Paleo I've wondered if it's 'legal'....

Pea Protein is a good way to go.. I use pea protein as an alternative to whey protein and when you add a little bit of water to it, it thickens.. So not only would you be able to thicken your soup/stew a little, you would also be adding some extra protein to your dish.. It doesnt really have a flavour, so it isnt something that will change the way the dish tastes..

My solution is to add rice, two hours or so before cooking finishes. If you hit the right ratio (1 rice to 3 water), most water will be absorbed by the rice.

If white rice is one of your acceptable foods, then here's a trick I learnt from Splendid Table podcast's interview with chef Andrea Reusing: "I use long-grain, aromatic rice called Carolina Gold Rice. I put it in a spice mill or coffee grinder for a second, and after a couple pulses, the pieces will be about one-third of their original size. We'll get a creaminess from the dust we made by grinding. About a quart of water is where I start. Depending on how much rice I've added and how fast it thickens, we'll just kind of adjust as we go. With soup, I find it's really hard to get the consistency you want far in advance, so even at the restaurant, we're always adjusting the texture of the soup at the last minute. I always start with less water than I think I need, so I can adjust by adding more." It works very well with stew also ... she used 1/2 milled rice for 6 cups of stock to make thick creamy soup. You can experiment yourself to get the right "stew" consistancy.