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1
Make Marcella Hazan's tomato sauce with onion and butter: Put the tomatoes and their juices in a saucepan with 5 tbsp of the butter and nestle in the onion. Set over a low flame and simmer steadily but slowly, uncovered, about 45 minutes, or until the fat separates from the tomatoes. Discard onion. Season to taste with salt and pepper and set aside.
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2
Toast the fennel seeds briefly in a dry skillet. Add the peppercorns, thyme sprigs, bruised garlic clove, and cream, and heat to just barely simmering. Cover, remove from heat, and set aside to infuse for 20 minutes or so. Strain the infused cream and refrigerate until ready to use.
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3
Melt remaining tbsp butter in a large skillet over medium heat. Saute the prosciutto a minute or two, then add the shallot, leek, and sliced garlic. Season to taste with salt, pepper, marjoram, and thyme. Continue sauteing until translucent, then add the fennel and toss to combine. Raise the heat to high and add the wine, allowing it to cook off completely before adding the broth and letting that cook off as well. Then stir in the reserved tomato sauce, cover, and simmer over low heat until fennel is tender, but not meltingly so.
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4
When the pasta is halfway done, remove the infused cream from the refrigerator and whip it to soft peaks. Stir it into the tomato-fennel sauce and simmer, uncovered, while the pasta finishes cooking.
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5
Combine the pasta with the tomato-fennel-cream mixture and cook together briefly over low heat, adding some of the reserved pasta water if necessary. Remove from heat and stir in the cheese. Divide among bowls, garnish with fennel fronds, and serve.
