I was inspired by my family's classic spinach dip for this recipe. I added slowly caramelized onions for a hint of sweetness and depth of flavor. This dip is delicious with veggies or even potato chips.
Melt the butter in a large skillet over medium heat and add the oil. Stir in the onions and sprinkle liberally with salt and pepper. Reduce the heat to medium-low and cook the onions, stirring occasionally, until caramelized, 30 minutes. Remove from the heat and cool.
Combine the mayonnaise, sour cream, cream cheese and cayenne in a mixing bowl and mix vigorously until smooth. Add the onions and thawed and drained spinach and mix well. Taste and adjust the seasoning with salt and pepper as necessary.