Remove the basil and parsley leaves, zest the lemon. Peel and finely chop the spring onions. Blend the spring onions, horseradish, 3 peeled and chopped garlic cloves, lemon zest, and basil and parsley leaves. Add 4 tablespoons of olive oil and blend until a pesto forms. Preheat the oven to 320°F (th.5-6).
Place the meat on a work surface, fat side down and the short side facing you. Season with salt and pepper, then spread the pesto over the meat. Cover with prosciutto slices and sprinkle with 1 oz of Parmesan. Roll the meat from the long side like a roll and tie it firmly with kitchen string.
Season the roast with salt and pepper and brush with oil. Brown it on all sides in an ovenproof skillet. Add a little water, cover with aluminum foil or a lid, and bake for 2 hours 30 minutes. Baste regularly with the cooking juices.
Prepare the polenta near the end of the meat's cooking time. Pour the milk and an equal amount of water into a saucepan. Add the bay leaves, thyme, black peppercorns, peeled and halved onion, and 1 peeled and chopped garlic clove, and bring to a boil. Turn off the heat, cover the saucepan, and let it infuse for 20 minutes.
Strain the aromatic liquid, bring it to a boil, and pour in the polenta while stirring with a whisk to avoid lumps. Cook for about 8 minutes, stirring until the polenta starts to thicken, then remove from heat. Stir in the butter and remaining Parmesan, and season with salt and pepper.
Transfer the roast to a cutting board, cover, and let rest for 15 minutes. Remove the string and slice. Sprinkle with pink peppercorns and serve with creamy polenta.