I served this utterly decadent dip at a party recently and guests couldn’t stop eating it and clamoring for the recipe. It’s sweet, rich, and not for the faint of heart, but it’s completely addictive. You’ll need to caramelize the onions before adding them to the slow cooker, an essential step—the onions become tender, nutty, and extra sweet, which makes a great contrast to the spicy Old Bay. It may seem like extra work, but the onions can sit in the pan and caramelize with just an occasional stir while you assemble the rest of the ingredients.
In a large skillet, melt the butter over medium heat. Add the onions and cook, stirring occasionally, until the onions are tender and caramelized, about 20 minutes.
Meanwhile, in a slow cooker, combine the cream cheese, mayonnaise, Old Bay, salt, cheddar, and mozzarella. Stir in the caramelized onions.
Cover and cook on high for 1 hour or low for 2 to 3 hours, until the cheeses have melted. Stir thoroughly before serving, making sure all the ingredients are well combined. Serve with the crackers.