Caramelize the onions. Melt the butter in a sauté pan over medium-low heat. Add the onion and cook for about 5 minutes. Decrease the heat to low and allow to cook for about 40 minutes, stirring occasionally, until the onions are very soft and translucent. Stir in the kale and turn the heat to medium high. Cook, stirring, for 2-3 minutes until the kale has wilted. Take off heat.
Make the dip. Preheat the oven to 400°F (200°C) and grease a 9" (22 cm) round (or similar sized) baking dish. Add the mayo, sour cream, 1 cup (113 g) of the mozzarella, salt, pepper, garlic powder and onion powder to the onion/kale and stir well. Spread into the baking dish and sprinkle with remaining mozzarella evenly on top.
Cover tightly in foil and bake 15 minutes, then uncover and bake for another 5 minutes, or until the top is bubbly.