Wash the leeks
Trim the root end and dark green tops from the Leek, keeping the white and light green parts. Slice them lengthwise if they are very thick, then rinse thoroughly under cold running water, separating the layers with your fingers to wash out any grit. Pat dry well so they soften rather than steam in the pot.
Prep the ingredients
Thinly slice the washed leeks if you have not already. Remove the casings from the Italian sausage by making a shallow slit down each link and peeling the casing away. Finely mince the Garlic, grate the Parmesan, and chop the Chives. Measure the Olive Oil, Butter, Salt, Black Pepper, Red Pepper Flakes if using, Dry White Wine if using, Chicken Broth, Shellbow Pasta, Heavy Cream, Frozen Peas, Lemon Juice, and Dijon Mustard if using so everything is ready to add on time.
Heat the pot
Set a large pot or deep skillet over medium heat and add the olive oil. Let it warm for about 1 minute, just until it looks loose and shimmery across the bottom.
Brown the sausage
Add the sausage to the pot and break it up with a wooden spoon into bite-size crumbles. Cook, stirring and scraping occasionally, until it is browned and cooked through, 6 to 8 minutes. If the pot has rendered a lot of fat, spoon off the excess and leave about 1 tablespoon behind for flavor.
Soften the leeks
Add the butter and sliced leeks to the sausage. Season with a small pinch of salt and a few grinds of black pepper, then cook, stirring often, until the leeks are soft, collapsed, and starting to look translucent, about 5 minutes.
Add garlic and wine
Stir in the minced garlic and red pepper flakes, if using, and cook for 30 seconds until fragrant. Pour in the white wine, if using, and scrape the bottom of the pot well to loosen any browned bits. Let it bubble for 1 to 2 minutes so the wine reduces slightly and the harsh alcohol smell cooks off.
Simmer the pasta
Pour in the chicken broth and bring it to a boil. Stir in the shellbow pasta, then lower the heat to a steady simmer. Cook, stirring often so the pasta does not stick to the bottom, until the pasta is just tender and most of the liquid has been absorbed, 10 to 12 minutes.
Finish and serve
Stir in the heavy cream, frozen peas, and about half of the grated Parmesan. Add the Dijon mustard, if using, and simmer for 2 to 3 minutes, stirring, until the sauce looks creamy and the peas are heated through. If the pasta seems too thick, loosen it with a small splash of extra broth or water. Take the pot off the heat and stir in the lemon juice. Taste and adjust with more salt and black pepper if needed, then spoon into bowls and top with the remaining Parmesan and chopped chives.
