Trim and discard root end and tough green top from leek; peel off and discard outer layer. Cut leek in half lengthwise and hold each half under cold running water, flipping layers to separate and remove grit. Thinly slice crosswise.
In a 5- to 6-quart pan over high heat, bring 4 quarts water to a boil. Add pasta and cook, stirring occasionally, until tender to bite, 8 to 10 minutes. Drain.
Meanwhile, in a 12-inch frying pan or 4- to 5-quart pan over medium-high heat, stir pancetta until fat is rendered and pancetta is crisp and brown, about 8 minutes. Transfer to a paper towel-lined plate. Discard all but 2 teaspoons fat from pan.
Lower heat to medium and add leek to pan; stir often until limp and slightly golden, 2 to 3 minutes. Stir in peas and cook until warmed through, 2 minutes longer. Reduce heat to low.
Whisk together 1/2 cup cream, egg yolks, cheese, salt, and pepper. Add hot pasta and cream mixture to pan with leek and peas; mix gently to coat. Stir in pancetta. If desired, add a little more cream to thin sauce. Pour into a large bowl and sprinkle with parsley.