For the Braised Pork Trotters:
Preheat the oven to 300°F. Split the bottom part of the pork trotters in half; this is the gelatinous part, so it will seep gelatin into the braising liquid. Score the flesh of the top part of the pork trotters. Roast the onion, celery and carrot in a large roasting pan, then add the pork trotters, thyme, rosemary, bay leaf, peppercorns, and veal stock to cover the trotters. Braise the mixture in the oven for 7 hours, or until the meat is falling off the bone. Remove the trotters from the braising liquid. Separate the skin and the meat and reserve separately. Trim the sinew and excess fat from the skin and trim it into neat rectangles. These will eventually form the outside of the roulade. The bones are not needed for this recipe. Strain, degrease and reduce the braising liquid to a thick sauce.
For the Roulade Sweetbread Filling:
Blanch the sweetbread in 1 gallon of boiling water with white vinegar and salt. Cook the sweetbreads until firm and cool them in ice water. Peel the outer membrane off, then dice the interior. Roast the sweetbread pieces in a saucepan until crispy. Add the reserved trotter meat and deglaze the mixture with sherry vinegar and reduced braising liquid from the braised pork trotters. Cook the trotter meat and sweetbreads together until the braising liquid and vinegar have glazed it. Put the roulade filling aside.
Pork Trotter Roulade:
Lay out a straight line of plastic wrap. Lay the reserved pork skin from the braised pork trotters on the plastic wrap. Top it with the roulade sweetbread filling. Top the filling with another layer of pork skin and wrap the plastic around the trotter with multiple layers of plastic wrap. Hold both ends of the plastic wrap and roll the trotter on the table until in comes together in the center nice and tight. Torque the cylinder into a roulade as you roll. Tie the ends of the roulade and chill it in an ice bath.
To Assemble and Serve:
Cut a ¾-inch slice of the trotter roulade. Pan fry the roulade in a sauté pan over a medium heat until it turns crispy. Once the first side is done, flip the roulade to the other side to crisp, and prepare the sunny side up eggs. Once the roulade slice is crispy, remove it from the pan and put the fried egg on top. We sauce the plate with sherry sauce, smoked pearl onions and mustard seeds. Season the dish with salt and pepper.