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1
Cut each leek in half lengthwise. On a cutting board, place 1 leek half cut-side down, then slice it lengthwise into very thin julienne strips. Repeat with the other leek halves. Set aside.
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2
In a large pot of boiling salted water, add the fresh (or frozen) shelled fava beans. Cook for 1 minute (or 2 minutes if frozen). Using a large slotted spoon, quickly remove the beans to a colander set over a large bowl. (The bean- blanching water will be used to cook the noodles, so keep the water at a simmer.) Rinse the beans under cold running water until cool. Using your fingers or a paring knife, carefully tear open the outer hull of each bean. Gently squeeze the fava beans into a small bowl; set aside. Discard the hulls.
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3
In a large saute pan, melt the butter over medium heat. Add the leeks, toss to coat with the butter, and cook, stirring occasionally, until the leeks are wilted and tender, 2 to 3 minutes. Add the morels, increase the heat to medium-high, and cook, stirring occasionally, until the liquid released from the mushrooms has been reabsorbed, 3 to 4 minutes. Reduce the heat to medium, add the fava beans, cream, salt, and pepper and cook, stirring occasionally, until the sauce has thickened slightly, 3 to 4 minutes. Turn off the heat. Taste and adjust seasoning as needed.
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4
Return the bean-blanching water to a boil. Add the tagliatelle, stir well, and cook, stirring occasionally, until the noodles are slightly undercooked. Using a ladle, remove 1/4 cup of the pasta-cooking water to a small bowl. In a large colander, quickly drain the noodles, then add them into the saute pan with the sauce. Turn the heat to medium-high and gently toss the noodles with the sauce until they are cooked to al dente, about 1 minute, adding some of the pasta-cooking water if the sauce becomes too thick. Garnish with the scallions, if using, before serving.
