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.
Serve with cut-up vegetables or chips, if using.