Core the tomato, roughly chop, and toss in a small bowl with the olive oil. Season with salt; set aside.
Bring the water and cream to a boil in a medium heavy-bottomed saucepan over medium-high heat. Add the butter and salt, to taste. Stir to melt the butter. Whisking constantly, add the polenta in a slow steady stream. Continue to whisk until all of the lumps have disappeared. Reduce the heat to low and gently simmer, stirring constantly with a wooden spoon, until the polenta is thick and creamy, about 20 minutes.
Fold in the corn, herbs, and the Parmigiano-Reggiano. Divide the mixture between 2 heated bowls. Top with the tomatoes and any accumulated juices. Tear the basil leaves into small pieces and sprinkle over; serve immediately.
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