1. 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.
2. 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.
3. 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.
4. 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.
5. 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.