Step 1Quarter the cabbage lengthwise. Put each quarter on its side and shred with a heavy knife by cutting the cabbage in thin slices, first crosswise, then around the core; remove the core and any thick ribs. You should have 11 to 11 1/2 cups shredded cabbage.
Step 2Heat 3 tablespoons of oil in a wide, heavy stew pan. Add the onions and cook over medium heat, stirring occasionally, until they begin to turn golden, about 18 minutes. Add the cabbage, and season with a pinch each of salt and pepper. Cook uncovered over medium heat (the cabbage will seem like a very large amount at first, but it will cook down), stirring often, until the vegetables are very tender, about 20 minutes. While the cabbage is cooking, bring a large pot of salted water to a boil.
Step 3Sprinkle the mixture with sugar and cook over medium-high heat, stirring often, until the vegetables are lightly browned, about 7 minutes.
Step 4Meanwhile, cook the noodles uncovered in the boiling water until al dente according to the instructions on the pasta packaging (check the pasta 1 to 2 minutes before the time indicated to make sure it isn't already done). Drain, reserving one-half cup of the pasta cooking liquid.
Step 5Add the noodles to the pan of cabbage. Toss over medium-low heat for 1 minute. Add about 3 tablespoons of the reserved noodle cooking liquid, or enough to moisten the mixture lightly, and cook until the noodles are well coated with the cabbage mixture, 1 to 2 minutes. Stir in the poppy seeds. Off heat, add the remaining tablespoon of olive oil.
Step 6Stir most of the pepitas and parsley with the pasta and cabbage, leaving about one-fourth of each to sprinkle over as garnish. Taste, and adjust the seasoning as desired. Serve hot, sprinkled with the reserved pepitas and parsley.