Start by cutting your onions into thin slices. This could take a while, but it’s definitely worth the effort.
Put the chopped onions in your slow cooker, then add the olive oil or butter, salt, and if you want, sugar. Mix thoroughly.

3. Put a lid on the pot and cook on low heat for approximately 8-10 hours. Stir every so often until the onions turn a deep golden color and caramelize.
30 minutes before the onions are ready, add minced garlic, thyme, rosemary, and white wine or vegetable broth. Keep cooking until the mixture smells good and is well mixed.

Meanwhile, prepare your pasta following the instructions on the package. Save a cup of the water from the pasta before you drain it.
Mix the caramelized onions with the cooked pasta, and add some of the pasta water if necessary to make a smooth sauce.
Mix the shredded Parmesan cheese into the mixture and add salt and pepper to taste.

Sprinkle with freshly cut parsley and serve right away.

Different Types and Advice
For a creamier result, add a bit of heavy cream to the onions before mixing them with the pasta. To make it more filling, add cooked mushrooms or cooked spinach. If you want more protein, add grilled chicken, cooked sausage, or shrimp to your pasta. For a vegan version, replace the Parmesan cheese with nutritional yeast and use olive oil instead of butter.