Thickening agent? * NO flour or cornstarch?

If you have any boxes of chili kits around, (I usually do) they have a bunch of seasoning packets in them. You can use the packet of 'Masa Flour,' after you mix it in water like you would with regular flour or cornstarch.

Cornstarch is easier to use and makes a smoother gravy/sauce. I dilute it in tepid water first,then stir while adding it to sauce/gravy. Flour is usually fried in a pan with butter first though can be added to a room temperature liquid without it clumping. A trick to thickening gravy is a drop or two of vinegar. You won't taste it .

If you have some instant potato flakes you can sprinkle a couple of measured tablespoons into the stew to thicken the sauce

Potatoes...do you have potatoes in your stew? Take some out and blender or mash it and add it back in. Or cook some rice and put it through a blender and add it in. Both will thicken it and both would taste delicious in stew.

The only other thing that comes to mind is tapioca flour, but that doesn't tend to be something you have just sitting around

Butter, and Arrow Root.

Add a potato or two depending on (cup up in cubes so it cooks faster).

Try gelatin

You don't have to have it thick, I don't and it taste perfectly fine.

barley, lentils, oats.