The meat came out very tough when I made this (but everything else is delicious!) Curious if anyone knows what I did wrong? I don't usually cook r...

I would emphasize “slow and steady cooking”. When I’ve let the liquid get so hot that it goes from simmer to boil, the meat ends up being tough.

Sometimes packaged stew meat can be so irregularely cut (large & small) pieces that it doesn't cook evenly. Stew meat requires a slow & steady braise/cooking in flavorful liquid to really make it shine. Next time I would make sure that stew meat pieces are close to the same size, enough liquid covers the meat, enough time is given for the tough cut of meat to cook in its liquid. I've made this recipe in September (I should post a comment on the recipe page - my bad) and it was a delicious. It's more a meat stew ragu that has a flavorful sauce that's best served on pasta, egg noodles, rice or mashed potatoes. Did you follow her instructions with stovetop cooking time, overnight refrigeration, and the next days simmering time? I would definetly reheat slowly on the stovetop making sure there's enough liquid (diced tomatoes/beef broth, red wine, etc.) to breakdown the tough/fibrous meat chunks. All is not lost and hope you enjoy this great recipe!

Thank you kat3029! Phew. :)

Thank you so much hardlikearmour! If I pop it back on the stovetop tonight and continue to simmer it do you know if it will tenderize, or is it a lost cause at this point?

I suspect it didn't cook long enough -- the tough cuts of meat like "stew meat" have a lot of collagen in them, and you you have to cook it long enough that it turns into gelatin to get the meat to be tender/fall apart.