In a large pot, bring the water to a boil and add the salt. Slowly and steadily add the polenta, stirring constantly with a wooden spoon to prevent any clumps. Reduce the heat to medium and let it cook slowly, stirring for about 30 minutes, until thick, smooth and creamy.
Meanwhile, in a large frying pan over medium-high heat, heat the olive oil. Add the beans, tomato, garlic and cook for a few minutes, until thick and saucy. Stir in the chopped rosemary.
Mix the bean sauce into the polenta evenly.
Line a rectangular cake pan with parchment or smear a spoon of neutral oil on the base and the sides of the pan. Tip the polenta and bean mixture into it, flattening the top and covering it neatly with a parchment sheet. Let it cool overnight at room temperature or refrigerate for 3 hours. The polenta will be set into a cake. Flip this over on a wooden board and cut into 5cm (2 in) squares.
In a medium frying pan, heat some neutral oil over medium-high heat and pan fry the polenta pieces until golden and crisp on the outside. Drain on a kitchen paper and continue until all the pieces are done. Serve warm, with drops of traditional balsamic vinegar.
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