Heat a large skillet over medium-low heat and add the butter. Add the sliced onion and the salt and toss to coat. Cook, stirring occasionally, until the onion caramelizes - this can take 45 to 60 minutes! You want to do it low and slow so the onion fully caramelizes. If at anytime the onion starts to burn, turn down the heat! Once the onion is caramelized, turned off the heat.
While the onion is caramelizing, make the eggs. To boil the perfect egg, place the eggs in a large pot and fill it with cold water. Bring it to a boil and let bubble for 1 minute, then turn off the heat and cover the pot. Let the pot sit for 15 minutes. After 15 minutes, place the pot in the sink and fill it with cold water to cool the eggs down immediately. You want the eggs to sit in cold water for 30 minutes or so, so you will need to dump the water and refill with cold water a few times. You can add some ice cubes too!
After 30 minutes, peel the eggs and place them on a paper towel. Slice them in half and gently remove the yolks.
In the meantime, while the eggs are boiling, heat a large skillet over medium-low heat and add the pancetta. Cook until crispy and all the fat is rendered, then remove the pancetta with a slotted spoon. Place it on a paper towel to drain excess grease.
Add the egg yolks, mayo, yogurt, mustard, caramelized onion, pepper and garlic powder to the food processor. Blend until smooth and creamy and mousse-like. You can taste and add additionally salt if needed. Scoop out the mixture and spoon it (or pipe it with a pastry bag) into the open egg whites.
Sprinkle on the pancetta and the fresh chives. Add the fresh oregano and serve!