If you don't have leftover ham, you could make this with deli ham (just cut it into strips, or else have them slice it thick and then you can dice it), with cooked bacon, with leftover turkey, or utterly meatless. It will still be good.
Stick your pasta bowls in a 200-degree oven to warm while you prepare the pasta (this may seem fancy and silly, but cream sauces congeal so unappetizingly on cold plates).
Begin by bringing a large pot of water to a boil and salt it heavily (I use about a quarter of a cup of kosher salt, but you could use half that amount of table salt).
In a wide pan over medium-low heat, melt two tablespoons of the butter and sauté the onion until it's very soft and starting to turn golden, around ten minutes. Meanwhile melt the other tablespoon of butter in a very small pan, and fry the breadcrumbs over medium heat until they are brown and crisp. Scrape them into a bowl when they're done, so they don't burn in the still-hot pan while you're not looking.
When the onions are done, dump the noodles into the boiling salted water and give them a stir. Now add the broth, peas, and ham to the onions, turn up the heat, and boil vigorously until the broth is reduced by about half (i.e. there is much less of it left in the pan), then add the half and half and continue to simmer the sauce while you season it with the lemon zest and juice and salt and pepper (taste it first-the ham may add enough salt).
Turn the heat under the sauce way down low while you drain the pasta, which should be done right about now. Stir the pasta into the sauce, stir in the parsley, and taste: does it need anything? A little salt or pepper or a little more lemon juice or zest or a splash more half and half? When it is perfectly delicious, serve it in the warm bowls and pass the cheese and breadcrumbs.
NOTE: Taste the pasta water -- it should taste as salty as seawater. And really, I can't stress this enough: if your cooking water isn't heavily salted, the pasta will stubbornly insist on remaining bland, no matter how much tasty love you lavish on it later. By contrast, well-salted pasta is almost always good, even if it is imperfectly sauced.