Add the pancetta to a large saute pan. Turn the heat to medium and cook until the fat is rendered and the pancetta starts to brown, then remove the pancetta from the pan and set aside. Add one of the onions and cook over medium-high heat until translucent, about 8 minutes. Add rosemary and thyme leaves, apples, fennel and half the celery. Cook, stirring occasionally, until soft and very aromatic, 8 to 10 minutes. Season with salt and stir in the pancetta. Transfer the mixture to a large mixing bowl and let cool.
Butterfly each turkey breast to make a wide, flat surface. Cover each half with plastic wrap and gently pound with a meat mallet to flatten.
Lay a large piece of caul fat out on a large, flat work surface. Lay about 10 bacon slices in the middle, slightly overlapping. Arrange a turkey breast on top. Lay half the stuffing mix on the turkey and roll it up, leaving the bacon unrolled and creating a neat even log. Wrap 2 pieces of bacon around each end, then fold the bacon that's underneath around the turkey. Wrap the caul fat in an even layer to create a smooth even log. Tie with twine to secure. Repeat with the second turkey breast.
Preheat the oven to 375 degrees F.
Put a large roasting pan on a burner, coat it with olive oil and heat over medium-high heat. Add the turkey rolls and brown on all sides. Remove the rolls from the pan and add the carrots and the remaining onion and celery. Season with salt and cook the veggies until they start to soften, 7 to 8 minutes. Add the wine and 1 cup chicken stock to the roasting pan along with the thyme bundle. Return the turkey rolls to the pan, cover with foil and roast for 25 minutes. Remove the foil and roast until an instant-read thermometer inserted in the center registers 160 degrees F, about 10 more minutes.
Transfer the turkey to a tray or platter and cover with foil to keep warm.
Put the roasting pan over 2 burners to make the gravy. Remove the thyme and skim the fat. Mix the flour with 6 tablespoons chicken stock and slowly whisk it into the roasting pan. Whisk in the remaining 2 1/2 cups plus 2 tablespoons chicken stock and bring the mixture to a boil. Reduce the heat and simmer until the mixture has thickened to a gravy consistency. Taste and season with salt, if needed.
Remove the twine from the roulades and slice into medallions. Arrange the roulades on a serving platter and serve with the gravy.