Heat the olive oil in a 4-quart saucepan over medium heat. Add the onions and cook, stirring occasionally, until softened, about 6 minutes. Add the tomato paste and red pepper flakes and cook, stirring to coat the onions, about 1 minute. Add the canned tomatoes, penne, vodka, 1 teaspoon salt and 3 1/2 cups water and bring to a boil over medium-high heat. Continue to cook, stirring frequently, until the pasta is al dente and the sauce has reduced by about half and thickened, about 12 minutes. Remove from the heat. Stir in the heavy cream, half the Parmesan and 1/2 teaspoon salt. Divide the penne among 4 bowls and garnish with the remaining Parmesan and torn basil leaves.
The success of this recipe is dependent upon the specific pasta shape suggested, so we don't recommend substitutions.
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