How to Make It
Bring the water to a boil in a medium nonstick saucepan. Add a pinch of salt, then slowly add the polenta in a steady, thin stream, stirring constantly, to avoid lumps. Cook over moderately low heat, stirring often, until tender and very thick, about 35 minutes. Remove the pan from the heat and stir in 1/2 cup of the mascarpone, the Parmesan and 1 tablespoon of the butter. Season with salt and pepper and keep warm.
Meanwhile, melt the remaining 1 tablespoon of butter in the oil in a large nonstick skillet. Add the onion and cook over moderately high heat until softened but not browned, about 4 minutes. Add the corn and jalapeño and cook until the corn is tender, about 5 minutes. Stir in the tomatoes, season with salt and pepper and cook until just warmed through and some of the tomato juices have been released, about 5 minutes. Stir in the basil and mint.
Stir the polenta; add a little hot water if it is too thick and spoon into bowls. Top with the tomato-corn ragout, garnish with thyme sprigs and serve hot. Pass more of the mascarpone cheese at the table.