Prepare the porchetta: Butterfly the pork loin by cutting it lengthwise and opening it flat like a book. Cover with plastic wrap and gently pound to an even thickness. Mix the garlic, rosemary, sage, thyme, fennel seeds, lemon zest, salt, and black pepper together. Rub the herb mixture generously over both sides of the pork. Lay the streaky bacon slices slightly overlapping on a large sheet of baking paper, forming a rectangle large enough to wrap around the pork. Place the seasoned pork loin at one end of the bacon and roll it up tightly into a cylinder, using the baking paper to help guide the roll so that the bacon wraps neatly around the outside. Tie the roll securely with kitchen string at 3 cm intervals. Refrigerate overnight, uncovered, to allow the exterior to dry out slightly.
Cook the porchetta: Remove the porchetta from the fridge at least 2 hours before cooking to bring it to room temperature. Preheat your oven to 220°C. Rub the outside with olive oil and a little extra salt. Scatter the chopped onions, carrots, and celery into a roasting tray and place the porchetta on top. Roast for 30 minutes until the bacon begins to crisp. Reduce the oven temperature to 160°C and continue roasting for a further 1.5 to 2 hours, or until the internal temperature reaches 63°C. Remove from the oven and rest for 20-30 minutes before slicing.
Make the herb oil: While the porchetta roasts, combine the olive oil, garlic, rosemary, sage, and fennel seeds in a small saucepan. Warm gently over low heat for 5 minutes to allow the flavours to infuse. Strain and set aside.
Make the roasting gravy: Transfer the porchetta to a resting board and place the roasting tray on the stovetop over medium heat. Pour in the white wine and scrape up any caramelised bits from the base of the tray. Add the chicken stock and simmer for 10 minutes. Strain the liquid into a clean saucepan and return to the heat. Mix the cornflour with a splash of cold water and whisk it into the gravy. Simmer until thickened to your liking, then season with salt and black pepper.
Serve: Slice the porchetta into thick medallions and arrange on a serving platter. Drizzle with the herb oil, spoon over some of the gravy, and garnish with fresh herbs.