Bring a large pot of water to a boil, add the potatoes and let cook until soft (about 30 minutes).
While potatoes are boiling, heat 2 tbsp butter in a large skillet. Reduce the heat to medium-low and add the onions, sautéing until the begin to soften (about 5 minutes). NOTE: Do not let them brown!
Reduce the heat to low, cover and let the onions caramelize, stirring occasionally (about 15 minutes).
When the potatoes are soft, add the remaining 1 tbsp butter, 3 tbsp sour cream, garlic, Sriracha, salt and pepper and whip with a whisk or hand mixer until smooth (about 2 minutes).
Add the avocado slices a few at a time, continuing to whip, until you have your desired color and flavor.
Top with caramelized onions to serve.