Step 1

Heat oil and butter in a large, deep skillet over medium-high until butter melts. Add sweet onion, yellow onion, red onion, shallots, and salt; cook, stirring occasionally and folding onions into each other, until barely softened and just starting to brown, about 12 minutes. Reduce heat to medium-low. Cook, stirring occasionally, until onions are caramelized, about 35 minutes, adding scallions halfway through cook time. If onions start to stick to skillet or brown too quickly, add a splash of water to skillet. Transfer mixture to a large bowl, and place cream cheese on top; let cool completely, about 30 minutes.

Step 2

Stir onion mixture until cream cheese is fully incorporated. Add sour cream, mayonnaise, garlic powder, and cayenne to onion mixture; stir well to combine. Season with salt and black pepper to taste. Cover and chill at least 3 hours or up to 8 hours or overnight.

Step 3

Let dip stand at room temperature 30 minutes. Serve with potato chips, toasted bread, and seasonal vegetables.