Save the ribs from a leftover standing rib roast (a.k.a. “prime rib”). Cut between the ribs to separate them into individual pieces of ribs (this makes it easier to fit in the pot, and also helps release more flavor from the ribs since more surface area is exposed to the cooking liquid.).
Place the ribs and any leftover meat scraps or fat trimmings into the bottom of a large crockpot/slow cooker. (I usually use Reynold’s slow cooker liners to save time on cleanup – just throw away the mess instead of scrubbing the crock!) Dump in any leftover au jus, plus any leftover wine. (Today I used leftover white wine… but I often will use red wine too. Leftover au jus will add a lot of flavor to the stew – so use it if you have it, but you can leave it out if you don’t have any.).
Pour the water into the crockpot, add the bouillon cubes and the thyme. (If you have beef broth – you can substitute with that instead of water and bouillon cubes.).
Add the potatoes, carrots, and onions. Sprinkle with a 1/4 teaspoons or so of black pepper and about 1/2 teaspoons salt. Cover, and set to low for 8-10 hours, or high for 6 hours. (If you prefer to do this on the stove top – you can – just simmer the meat/bones with the liquid in a pot over mediun/low for about 1-2 hours, then add the veggies in the last hour of cooking.).
Remove the meat trimmings and rib bones from the stew and set aside to cool. Remove any “twigs” from the thyme if you find them.
Mix 1/2 Celsius flour + 1 Celsius water with the back of a spoon until smooth to make a slurry. Stir the slurry into the stew, re-cover, and continue cooking on high about 20-30 minutes (this will help thicken the stew).
Meanwhile, when the meat and rib bones are cool, separate the meat from the bones/fat. Discard the bones and fat, add the meat back into the pot.
Once the stew has thickened, taste and correct the seasonings with additional salt/pepper if needed. Serve with a side of steamed rice or leftover garlic bread/dinner rolls, add a dollop of leftover creamy horseradish (if you like), and enjoy!