Thinly slice onions and add to a large cast iron skillet or dutch oven, along with some olive oil. Cook on low, stirring often until onions are super soft, about 25 min.
Add in parsley, garlic, peppercorns, red pepper flakes and bay leaves. Cook for about 5 min. Then add in white wine and cook until wine is reduce in half.
Add in water, gruyere and parmesan rinds. Cook for about 2 hours stirring often to make sure parm doesnt stick to bottom as it melts. Taste test and add some lemon juice at the end. you can strain it to remove all the herbs/ garlic shells and rinds. Enjoy!
If pan gets tough with the rinds at the end, add some water to your pot and heat on low. Once the pan is warm, you should be able to scrape off the excess parmesan and more easily clean your pan.
