Prepare a creamy sauce with the sweet potatoes: simmer the potatoes and then mash them with a potato masher. Put the potatoes in a saucepan together with a knob of butter and some milk. Use a whisk to thoroughly blend everything together. Add salt and pepper, and a little more milk to make the mixture softer. Hand whisk it again.
Clean and fillet the brill. Season each fillet with a sage leaf, a little salt and pepper. Cook the fillets in a pan with a knob of butter.
Cut the focaccia in half. Spread the sweet potato cream on the focaccia base, then add freshly grated Parmigiano Reggiano. Continue with the brill fillet adding a few more fresh sage leaves. Finally, top the sandwich with slices of Prosciutto di San Daniele PDO.
