I made some beef stew in a crock pot. It has been cooking on low since the early AM hours, so it will be ready this afternoon/evening.
The stew looked watery, so I added a little bit (maybe too much? About 1/4 cup) of cornstarch. This thickened it up wonderfully, but now it tastes like cornstarch. Can I do anything to fix it?
Boil a little water with some beef bouillon, or if you have a can of beef broth, boil it down to concentrate it. I’d also add a splash of coffee. If you don’t have any bouillon or broth, a splash of coffee, maybe a bit of red wine and salt. Taste. Add more salt if needed. You could even pull out a bit of the meat and try boiling that with the salt and coffee (and some water of course) to try and extract some of the meat flavor to put back into the stew.
A) WAAAAAAY too much cornstarch. You’re gonna have Jell-O when it cools! 
B) Best way to add cornstarch? Get the pot up to a rapid simmer. Mix about 2 tablespoons of cornstarch in about 1/4 cup of COLD water until it’s lump-free. Drizzle the cornstarchy water into the simmering stew. Give it about 15 minutes to start to thicken
C) Add some liquid and let the cornstarch cook. It’ll lose the “raw” taste eventually but you’ll need to thin it down.
Fenris
What Fenris said, 1/4c is probably triple what you needed. Also when mixing the cornstarch into the COLD water, use your finger as it is the best way to make sure all the lumps are out.
You made a pot of beef stew. What Iwould do is turn it into two pots:) I’d get some more veggies and meat, pressure cook em then divide the stock you made, add some red wine to taste and simmer. Beef stew freezes well.
Well, now I know about cornstarch amounts. Thanks! 
I used the beef boullion advice, I didn’t have anything else on hand. I also cooked it for a couple more hours, and IMHO it tasted great! My SO and my sister both ate some, and they liked it too. The stuff left at the bottom was rather thick, but we were already full by then.
Personally I hate the taste imparted to dishes by using any flour to thicken it late in cooking. If using flour in a meat stew I add it when I brown the meat by coating the meat. Otherwise I take the lid off later and let the stock reduce.
My trick for all stews, sauces and gravies which haven’t reduced sufficiently is to thicken them at the last minute with instant mashed potato. I only keep it in the pantry for this purpose - due to the fact that it’s virtually tasteless it adds no flavours to anything. Add it a spoonful at a time and stir through until the desired consistency is reached and then serve.
Tastes like cornstarch? Make it taste like something else! Such as:
Mrs. Dash
Chopped garlic
A shotta hot sauce
chopped rosemary
oregano
ketchup
red wine
bourbon
curry
chopped onion
a bit of orange juice
Pick one, pick three, pick five!
–Nott