Preheat the oven to 425 degrees. In a large roasting pan, scatter the onion slices and lay the bacon slices over them, then slide into the oven.
Roast for 20-30 minutes, stirring occasionally, until the bacon is ultra-crispy. (The amount of time will depend on the thickness and fattiness of your bacon.) While the bacon and onions are in the oven, bring a large pot of well-salted water to a boil.
Remove the pan from the oven. Use tongs to remove the bacon to a plate and pour the excess bacon fat into a small bowl.
Boil the gnocchi according to package directions, drain, add to the roasting pan, and stir with the onions, adding 1-2 tablespoons of bacon fat back into the pan to coat. Return the roasting pan to the oven for 5 minutes.
Stir the vinegar, mustard, salt, sugar, pepper, and celery seeds together and add to the gnocchi and onions, scraping up any brown bits on the bottom of the pan.
Roughly chop the bacon and add to the pan with two-thirds of the parsley and toss well, then taste for seasoning, adding additional bacon fat and/or spices as you see fit. Move to a serving bowl, sprinkle with remaining parsley, and serve immediately.
