Caramelized Onion Spinach Dip

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5 tablespoons unsalted butter

2 tablespoons vegetable oil

1 red onion, thinly sliced

1 yellow onion, thinly sliced

Kosher salt and cracked black pepper 

1 cup mayonnaise

3/4 cup sour cream

4 ounces cream cheese, softened

1/2 teaspoon cayenne pepper

One 10-ounce box frozen spinach, thawed and drained

Cut-up vegetables, for serving, optional

Chips, for serving, optional