Add the garlic to the onion-carrot mixture in either pan and cook 1 minute. Add the tomato paste, thyme and bay leaf, and sauté for 2 minutes. Transfer the vegetable mixture to the slow cooker and, off heat, add half the wine to each of the two skillets the vegetables were cooked in. Bring the wine to a boil and simmer until it is reduced by about three fourths.
When the wine is reduced in each of the skillets (you should have a total of 1 cup from both skillets) add it to the slow cooker along with the stock. Put the lid on the slow cooker and cook on low for 8-10 hours or until the meat is very tender and falling off the bone.
Transfer the ribs to a platter with tongs and let stand until they are cool enough to be handled. Meanwhile, strain the sauce into a bowl. Discard the solids and return the liquid to a large saucepan. Skim off any fat that floats to the surface (or use a fat separator). Whisk together the flour (judge how much to use, somewhere between 3-5 tablespoons) and 1/3 cup water. Bring the cooking liquid to a boil and add the flour mixture in a steady stream, whisking. Bring the sauce to a boil, and simmer for 5 minutes. Whisk in the mustard, lemon juice and salt and pepper to taste.
Meanwhile, discard the bones and trim any excess fat and gristle from the rib meat. Add the ribs to the pot and cook gently just until heated through. To serve: arrange some rib meat on each plate, spoon some of the pan sauce over and top with some of the Horseradish Sauce.
Note: This is recipe tastes even better a few days after you make it. And, if you make it ahead, it is easier to remove the fat. Here is what you do: After the ribs are tender, transfer them to a bowl and strain the cooking liquid over them, discarding the vegetables. Let the mixture cool, cover and chill overnight. The fat will solidify on top and it is very easy to scrape off and discard. Then gently reheat the ribs and the liquid in a large saucepan until the ribs are warm. Strain the liquid and set it aside. Discard the bones and trim the excess fat and gristle from the ribs. Thicken the liquid following the instructions in the recipe, add the ribs to the thickened liquid, heat and serve.
For the Horseradish Sauce:
Place the horseradish in a small strainer set over a cup and press until the horseradish is quite dry. Measure and set aside 1 tablespoon of the drained horseradish and 1 tablespoon of the liquid. Strain more if you don't have enough.
Whisk together the sour cream, reserved horseradish and liquid, lemon rind, and salt and pepper to taste.
Makes about 1/2 cup