For the lime pepper oil, squeeze the limes, mix with the olive oil and stir in the herbs. Season to taste with salt and pepper.
For the polenta, peel and dice the onion and sauté in the olive oil. Pour in milk and soup, add rosemary and thyme and simmer for 3 minutes. Strain. Return the liquid to a boil and stir in the polenta. Simmer over low heat until polenta is tender. Season with salt, pepper and nutmeg and fold in the grated Parmesan.
Spread the polenta mixture on a baking sheet and cut into rectangles. Fry briefly in hot olive oil on both sides until golden brown.
Salt and pepper the branzino fillets and sprinkle with lime juice. Heat a mixture of butter and olive oil in a frying pan and fry the fish fillets in it on the skin side for 3 minutes.
Arrange the fillets of branzino and the fried polenta on warmed plates and drizzle with the lime-pepper oil.