Starches, whether it is flour, corn etc... require significant heating(as in bring to a boil) to really thicken. So dumping it in won't do that much. It will do a little though. If you must, choose cornstarch, it thickens better and won't leave a pasty taste. Plus it gives a nice "shine" to sauces. To use cornstarch, make a slurry with the starch and water and stir it in.
Cheese will thicken the sauce as well, this might be the best option.
I don't use freeze dried stuff, but I have noticed all my friends who do, they don;t add as much water as the package says. Maybe less water and the aforementioned problem will be a non-issue.
I find the best way is to make your soup, stew, chili or whatever at home and when it gets to the consistency you like then dry it. At camp add a slight bit less water than what the recipe calls for (you can add more if it is still too thick). This way you do all the simmering at home and on the trail you just rehydrate and reheat.
I like sauces that are hearty... like this....
which is probably why FBC doesn't appeal to me as much as my own way of doing things.
Potato starch will work, as does cutting back water and or adding instant potatoes or cheese. In most cases I make the sauce part of the meal (integrated). To make a pack of say Alfredo sauce to pour over pasta separately, cooking it in a Ti mug would be the best choice - so it can have the heat and time needed for the thickeners to work.
Freezer Bag Cooking, Trail Cooking, Recipes, Gear and Beyond: www.trailcooking.com
I often use potato FLOUR to thicken sauces and soups as it doesn't require much, nor does it require boiling to create a thicker consistency. It is similar to potato flakes (instant potatoes) but without all of the amendments. It has a slight potato flavor, but generally not enough to make you say "Wow, POTATO!"
YMMV. Viewer discretion is advised.
Okay, everyone can laugh, but I use masa flour that comes with the chili seasoning mix I use as a starter for chili. It's always left over and works well for thickening things. Has no appreciable taste.
Loc: Eastern MA, USA
Hmm-My first reply didn't get posted for some reason.
I'm not laughing at the corn flour as thickener suggestion. I use it in chili. Other easy thickeners that are actually pre-cooked include finely ground (dry) bread, cracker, or cookie crumbs, or infant cereals. Consider pulverized ginger snaps for thickening an Asian or German-inspired sauce (Like Teriyaki or Sauerbraten). The upside of using infant cereals for thickening, in addition to their being pre-cooked, is that they are highly fortified with iron. Crackers will add a significant amount of sodium, so unless you feel you are sweating heavily and need to replace some, be careful.