Heat a pan over medium heat and when hot, add the bacon. Cook until the fat is rendered and the bacon is crisp, about 5-10 minutes. Remove the bacon to a paper towel lined plate, leaving the bacon fat in the pan.
Add the onions to the bacon fat and toss well to coat the onions in oil and cook, stirring occasionally over medium high heat. After softening, stir every 5 minutes as they caramelize, about 30 more minutes. Towards the end you may need to stir more often, and cook to desired color/caramelization, another 5-10 minutes. If things seem like they are burning or sticking too much, add a couple tablespoons of water to the pan and stir well. Set aside to cool.
While the onions are caramelizing, preheat the oven to 325ºF and grease a pie plate with some oil and set aside.
Whisk eggs together with salt and pepper and then add the cooled onions, bacon, Gruyere, and nutmeg, stirring to combine evenly.
Pour into a greased pie pan and bake for 40-45 minutes, or until set, the internal temperature should be 172ºF.
Allow the quiche to rest for at least 15 minutes before slicing and serving. It must be cooled to be sliceable.
Notes
Nutrition as 6 servings rather than 8: Calories: 626 cal, Carbs: 8, Fiber: 1g, Fat: 56, Protein: 24g