On a lightly oiled baking sheet spread warm polenta about 3/4-inch thick and cool to room temperature. Polenta may be made 2 days ahead and chilled, covered. In a large non-stick skillet heat 1 tablespoon oil over moderately high heat until hot but not smoking and brown sausage. Add water and cook sausage, covered, turning occasionally, until cooked through, about 10 minutes. Remove lid and cook mixture until any remaining water is evaporated. Transfer sausage with tongs to a bowl and reserve fat remaining in skillet. Cool sausages slightly and slice diagonally. Cut polenta into 1 1/2-inch diamond shapes or squares. In skillet saute polenta in 2 batches in reserved fat over moderately high heat, turning occasionally, adding additional oil to skillet if necessary, until golden on both sides. Transfer polenta as cooked with a slotted spoon to bowl with sausage and cover. In skillet heat remaining tablespoon oil over moderately high heat until hot but not smoking and saute broccoli rabe with salt to taste, stirring, until just wilted. Add garlic paste and saute, stirring, 1 minute. Stir in remaining ingredients and simmer, stirring, until tomato paste is incorporated and sauce is slightly thickened. Add sausage and polenta and cook until heated through. Serve polenta sprinkled with Parmesan.
In a heavy saucepan bring water and salt to a boil and gradually whisk in cornmeal in a thin stream. Cook polenta over moderately low heat (it should be barely boiling), stirring constantly, until very thick and pulls away from side of pan, about 40 minutes for cornmeal and about 15 minutes for instant polenta. Remove pan from heat and cover to keep warm. Stir polenta just before using. Polenta will keep warm, covered, about 20 minutes. Makes about 3 cups.