Slice the onions thinly and place them in the slow cooker with olive oil (or butter), salt, and optional sugar. Stir well.
Cover and cook on low for 8–10 hours, stirring occasionally, until the onions are deeply golden and caramelized.
About 30 minutes before the onions are done, stir in garlic, thyme, rosemary, and white wine (or broth). Let cook until fragrant and combined.
Meanwhile, cook your pasta according to package directions. Reserve 1 cup of pasta water before draining.
Toss the caramelized onion mixture with the pasta, adding reserved pasta water a little at a time to form a silky sauce.
Stir in Parmesan cheese, season with salt and pepper, and garnish with fresh parsley.
Serve hot with salad and bread on the side.
