Preheat the oven to 350°F. Season the short ribs with salt and pepper. Place the ribs in a large roasting pan. Scatter the onions, celery, carrots, tomatoes, garlic, bay leaves, thyme, parsley, and peppercorns over them. Make a second layer of ribs if necessary. Pour in enough stock to cover the meat. Place the pan on the stovetop over medium-high heat and bring to a boil. Cover the pan tightly with aluminum foil and bake for 3 hours, or until the meat is very tender and falling off the bones.
Using a slotted spoon, remove the ribs from the broth while still warm. Reserve the broth and vegetables. Remove the bones, fat, and gristle, leaving the meaty portion in 1 large strip for the terrine, if possible.
To assemble the terrine, line a 9 x 5-inch loaf pan with plastic wrap, leaving an overhang of several inches. Pack the short ribs lengthwise into the pan with some of the cooked carrots as tightly as possible. Make several layers. Fold the plastic wrap over the loaf pan to cover. Place the terrine in a dish or tray to catch spilling juices. Place a second 9 x 5-inch loaf pan over the terrine to compress it. Tie the loaf pans together with string at each end. Pour off any juices. Refrigerate for at least 24 hours.
Strain the cooking broth through a fine-mesh sieve, pressing on the ingredients. Cover and refrigerate. Prepare the herb vinaigrette and chill.
To serve, remove the terrine from the refrigerator, cut the strings, and remove the top loaf pan. Open the plastic wrap and invert the terrine onto a cutting board. Immediately slice it into 1/2-to 3/4-inch-thick slices, using a sharp knife and a broad spatula.
To serve cold, place each slice on a plate. Sprinkle lightly with kosher salt and ladle the vinaigrette alongside.
To serve warm, preheat the oven to 350°F. Place cold slices on individual ovenproof plates and reheat for 5 minutes. Remove the chilled layer of fat from the cooking broth.
Reheat and ladle several tablespoons of broth over each slice of meat. Sprinkle lightly with kosher salt. Ladle the vinaigrette alongside.