I skim fat from my stews and then thicken it with a roux or cornstarch. Is there a way to directly thicken with the floating fat?
If you want to use the fat to keep the flavor, or because you are keeping a diet where fat is considered more beneficial than carbohydrates, there are several ways.
First, you can skim the fat, make a roux with the skimmed fat, then add it back.
Second, you can skim the fat, add a little bit of broth and an emulsifier, whisk until you have a nice thick emulsion, then add it back.
If you use a straciatella style thickening, the fat eyes will frequently disappear between the rags, especially if you can get them to break up very small.
But if you are asking because you want to save some work, I don't think there is a good way. Generally, if you use enough flour or other starches at the frying at the start of the stew (including tomato pastes and such), the fat will bind with the starch instead of floating. But it seems that your recipe is for a thin stew, and the fat continues to float there.
What you are actually doing is thickening with the starch from the flour or cornstarch; the fat is only helping it not clump.
While there do exist methods of thickening with fat (emulsions such as beurre monte sauces, or even mayonnaise), they don't generally apply in a practical manner to the fat on top of a cooked down soup or stew.
I think what you are getting that is you'd rather not have to thicken the stew in such a complex way. It's actually very easy to thicken with cornstarch, all you do is mix cornstarch with cold liquid and then pour it into the hot stew. If you put cornstarch directly in it will clump because of the heat. It doesn't take much water or milk, but if you don't want to dilute your stock then get a couple of cups of stock out and cool it in the fridge, then add the cornstarch to that.
Note you can use flour and not cornstarch, however raw flour may add some off flavors. If you are going to use flour then still fry it off.
Note that you will lose some flavor by not making a roux, and you may want to skim some fat off anyway to keep the stew from being greasy.