This is based on a dish of penne, tomato, cream, and Swiss cheese I had when I was a student in Fiesole many years ago. My recipe gets an assist from Marcella Hazan, whose Tomato Sauce with Onion and Butter is incorporated herein. Eliminate the penne, tweak the amounts of dairy, and play around with the proportion of leeks to fennel, and you've probably got the makings of a pretty good gratin as well. —Chris Hagan
unsalted butter, divided
yellow onion, peeled and halved lengthwise
canned chopped tomatoes and their juices, preferably San Marzano variety
garlic cloves, 1 bruised but unpeeled, the other peeled and sliced thinly
a few thyme sprigs
a few peppercorns
prosciutto, sliced into thin strips
shallot, sliced thinly
leek, white and light-green parts only, well-washed, dried, and sliced into1/8-inch half-moons
fennel bulbs (1-1.5 lbs trimmed), halved and julienned lengthwise (reserve some fronds for garnish)
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.
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.
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.
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.
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.